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KAI RYSSDAL: This radio program has been on the air since 1989, 20 years. A pretty good run by most standards. But Marketplace can't hold a candle to "Guiding Light." Today, CBS aired that soap opera's final episode. And a 72-year run -- first on radio and then TV -- came to an end.

Nate DiMeo looks back at how it started. And why it faded to black.

"Guiding Light" radio jingle: Put new Duz in your washing machine. Finest suds you've ever seen!

Nate DiMeo: If you've spent ever your afternoon waiting to find out whether a doctor is actually a doctor or is in fact that doctor's evil twin, you pretty much have one woman to thank.

"Guiding Light" radio introduction: "The Guiding Light," created by Irna Phillips.

In the early days of the Depression, Irna Phillips left her job as a Chicago school teacher for a career in radio, at a time when the new industry was just sorting itself out.

Ron Simon is a curator at the Paley Center for Media.

Ron Simon: I don't think radio knew there was going to be an audience in the afternoon. It was Irna Phillips that envisioned a new format.

We all know the format now: the serialized domestic melodrama, the soap opera. Phillips not only pioneered the opera part, which had huge audiences of women weeping into their wash tubs, she invented the soap part too.

Simon: Irna was very savvy about the marketing possibilities. If there was a wedding going on, she tried to find some type of company that would help sponsor that wedding and then be mentioned. She understood what product placement was all about.

And with her hit program, "Guiding Light," Phillips entered one of the longest and most successful marriages between mass entertainment and mass consumption in history. A partnership with Procter & Gamble that took Phillips's format to TV.

"Guiding Light" TV introduction: "The Guiding Light". This portion is presented by Cascade with sheeting action and by Zest, the deodorant bar that leaves you feeling cleaner.

Since the late 1930's "Guiding Light" has been produced by P&G. Now, that's not sponsored by, but produced. It's a relic from the earliest days of radio and TV. Networks would lease air time to corporations, and they'd be in charge of making shows good enough to land an audience. But that audience has become harder and harder for soaps to capture.

Ellen Wheeler: Well if you look around the office, there's not much left.

Ellen Wheeler is the executive producer of "Guiding Light." Well, she used to be anyway.

Wheeler: There's a phone and we answer the phone to the fans when they call and say, "please don't let the show go off the air."

She also used to be a soap star in the 80s, on "Another World," another Procter & Gamble show. And the 80s were a great time to be in soaps. There were more than a dozen on the air. But it also may have been the beginning of the end for the genre.

Wheeler: If you actually go back and check the ratings, what we think of as the height was actually the beginning of a very slow decline.

That's because, fewer and fewer women were staying home in the middle of the day. Then throw in DVRs and mobile devices and a gajillion cable channel all chipping away at the ratings. And CBS -- the network that had carried "Guiding Light" since the Truman administration -- kept asking the show to tighten its belt. Two seasons ago, Ellen Wheeler and her bosses at P&G, realized the belt was about to snap.

Wheeler: We ran the numbers every which way: upside down, backwards, sideways -- we knew we had certain budget constraints that we had to hit that could not be met shooting in the old style.

So they tried shooting in an entirely different way -- one that was more shaky-hand held documentary than classic soap: cheaper cameras, smaller crews, fewer actors, fewer sets. But the same delightfully preposterous plot twists.

Lizzie Spaulding: Alan, no. It doesn't have to be this way put the gun down. Leave Jonathan alone and give us back Sarah. You're only doing this because you want Sarah, but you don't needed her anymore, because that's your baby!

Alan Spaulding: My baby?

Not every one was happy with the new "Guiding Light." One critic even accused Wheeler of trying to make the show so bad, CBS would have to cancel it. She says nothing could be further from the truth.

Wheeler: We gave the audience two more years of "Guiding Light" making these changes, we also brought something new and fresh to the whole idea of soap operas.

And she says there's going to have to be a whole new way of making soap operas if there are going to be any soap operas at all. The money just isn't there. At one point, Procter & Gamble had 13 soap operas on the air. As of today, there's just one: "The Young and the Restless," which is looking a little bit old these days.

I'm Nate DiMeo for Marketplace.