LOS ANGELES  When Ben Silverman, co-chairman of NBC Entertainment and its associated production studio, addressed a meeting of reporters in July, he confidently predicted that “Heroes,” the network’s biggest prime-time drama, was going to get even bigger.

Although the series had been off the air since the writers’ strike last year, Mr. Silverman said that Tim Kring, the creator of “Heroes,” had successfully addressed the problems that had plagued the show’s second season  confusing plotlines, scattered characters, questionable leaps in logic, all of which led to a shrinking audience from the critically acclaimed first season.

“We’re really confident, and we’re excited to see the audience respond and come back,” Mr. Silverman said, “and we think it’s going to be its biggest year ever.”

That has not happened. With ratings for “Heroes” down 20 percent from those of a year ago and regular third-place finishes in total viewers for its time slot, NBC took action last week, midway into the show’s third season. Two co-executive producers were fired: Jeph Loeb and Jesse Alexander, who also served as senior writers and oversaw much of the script development.