NBC has added a new 10 p.m. drama with glimmerings of promise in “Parenthood” ; a comedy reality show that has performed more than respectably in “The Marriage Ref”; and it has restored a long-running hit, “Law & Order: Special Victims Unit,” to 10 p.m., where the drama has resumed being the top show at that hour. (Last fall, it dropped to last place after being displaced to 9 p.m. to make room for Mr. Leno.)

As far as the late-night picture goes, “it’s as if a collective erase button was pushed,” said Robert Thompson, professor of television at Syracuse University, “with the usual suspects back in their usual locations — except Conan is gone.”

A critical result from NBC’s point of view is that Mr. Leno has drawn the biggest audiences in most age groups almost every night since returning March 1. That performance has defied what Mr. Adgate described as expectations — built of resentment by younger viewers over what happened with Mr. O’Brien — that Mr. Leno might not automatically resume his winning ways.

That he has is not the best news for David Letterman on CBS, who grabbed the leadership during Mr. O’Brien’s brief run, nor for “Nightline” on ABC, which also enjoyed a competitive boost.

But the situation is not exactly as it was before NBC disturbed the late-night universe. Mr. Leno has not been unaffected by the turbulence. His ratings lead is clearly down from where it was a year ago.

Measured against the same period beginning in March 2009 (and, to make the comparison as fair as possible, excluding one huge night when President Obama was his guest in 2009 and the abnormally big week he enjoyed his first week back this year), Mr. Leno is down about 18 percent in viewers, and slightly more among the 18- to 49-year-old viewers that matter most to many advertisers.

With those anomalies factored out, Mr. Leno is now averaging about 4.4 million viewers a night, down from 5.37 million in 2009. His rating among 18- to 49-year-olds is about a 1.15, down from a 1.5. By contrast, Mr. Letterman is down to about 3.54 million viewers, from 3.76 million last year, and his rating in 18- to 49-year-olds is now about a 0.95, down from a 1.0. “Nightline” is down to 3.61 million viewers from about 3.85 million last year and to a 0.9 rating in the 18-to-49 age group from a 1.1.