Producer Barney Rosenzweig knows that he's got a lot of emotional investment in his long-running, Emmy-winning "Cagney & Lacey."

So much, in fact, that when his beloved series was rumored to be in danger of cancellation last spring by CBS, "I spent a lot of time anticipating the end ... I went into a real tailspin for about three days."

And in the same way that a man eventually must face his own mortality, Rosenzweig finally admitted that life with his favorite show would inevitably come to an end -- if not then, possibly at the close of the 1987-88 season, surely by '88-'89.

"It has become an obsession, unique and extraordinary," he said last spring. "I feel this debt to it, because it allowed me to become what I always wanted to be. I do every edit, every costume, every music cue. I concede to a sort of obsessive quality. 'Cagney & Lacey' has a level of consistency that no other series has. I'm proud of that."

He's still proud, still intense when he talks about the series he created and which, he says, commands one of the highest advertising rates of any CBS series. Ad rates for "Cagney & Lacey" are steeper than those for some higher-rated shows because it attracts affluent, educated women between 25 and 50.

Last week, in town for an all-night, 17-hour marathon screening of "Cagney & Lacey" episodes at AMC's Academy 8 Theater in Greenbelt, Md., Rosenzweig talked about the series and his plans.

"'Cagney & Lacey' is an emotional experience for me. It has lifted me out of the mire of obscurity. It has brought an incredible amount of benefit to my life, light to my life, for which I am very grateful. The accumulation of capital has never been something that has particularly interested me, although 'Cagney & Lacey' has made me ... by my standards, somewhat financially independent. But that wasn't the goal. I was always more interested in renown, more interested in glory, in power. That's probably why I'm always so comfortable in Washington, the power town.

"But the emotional investment in 'Cagney & Lacey' is exceptional ... I know the chances are that I will never have that again, that that is a once-in-a-lifetime experience. And so my thought was that I should do something different, do it differently. What I'm hoping to do is to enlarge my scope, ultimately to invest time and interest in people and turn them into Barney Rosenzweigs of their own."

(One of those beneficiaries is "Cagney & Lacey" writer Georgia Jeffries, with whom Rosenzweig is going into partnership to create a new series, possibly to be set in Washington.)

"We're very close," he continued. "Sharon Gless is my best friend. And Tyne Daly doesn't care -- I mean, she already has a husband ... If Brigham Young were president, I'd ask them both to marry me." (Rosenzweig is married to Barbara Corday, president of Columbia Pictures Television.)

"But 'Cagney & Lacey' has brought to me about everything I ever wanted, and it's going to be gone very soon -- the end of this year, possibly another year. I would, if I could, bring closure to the show. I already know what the last episode would be. But I've been asked by Orion, which owns the show, not to do it. They're afraid that if we close the show, it would hurt syndication. It's a financial decision ... So when they asked me -- they didn't tell me, they asked -- I have to honor it."

That leaves Rosenzweig to "reexamine a lot of my goals ... I'm 50 now. Simple metaphor: the 50-yard-line, half way. I'm at halftime. I am now exploring my game plan. I feel like the coach of a football team at the half-time and the score is now 28-to-nothing in our favor. But the quarterback is tired. I know I'm not going out there to score another 28 points ... I know I will do things differently in the second half and it has nothing to do with second-guessing the first half. The first half was great, but it was the first half.

"I'm not afraid of failure, in terms of commercial success or failure. I have failed in the past and survived. I don't mind it; I'm not afraid of it. My definition of success is triumph over failure, so you have to fail in order to succeed."

In truth, Rosenzweig's favorite show also failed before succeeding: It was cancelled twice, then won the Emmy for Best Drama Series in 1985 and 1986. Two other actresses (Loretta Swit and Meg Foster) played Christine Cagney before Gless took the role. Gless won the Emmy as Best Actress in 1986; Tyne Daley has won it three times.

In the fall, as Rosenzweig looks ahead to the second half of his career, things will change on "Cagney & Lacey," as well. The series, having marked its 100th episode last March, began production for the '87-88 season on Monday.

Its two-part season ender found Christine Cagney admitting that, like her late father who was also a cop, she was an alcoholic. This fall, as Cagney begins her own growing-up process, the older son of her partner, Mary Beth Lacey, will talk of joining the Marines, having become enamored of Lieut. Col. Oliver L. North during the televised Iran-contra hearings. BY FRED SWEETS -- THE WASHINGTON POST tv.capt,28p Rosenzweig: "'Cagney & Lacey' has brought to me about everything I ever wanted."