By Doug Young

LOS ANGELES (Reuters) - Who says the second time is never as good as the first?

Certainly not the major U.S. broadcast television networks, which are beginning to discover the low risk and high potential rewards of rebroadcasting popular cable TV shows to a larger broadcast audience during the slow summer months.

Such programs cost a fraction of what the networks pay for their own shows because the cable companies bear most of the development costs. Furthermore, the shows are cheaper because networks typically broadcast the second run, rather than the more costly first run.

In ground-breaking moves, earlier this summer both NBC and ABC announced deals to rebroadcast popular cable shows on prime network TV. NBC took "Forensic Files" and ABC grabbed "Monk."

The practice of U.S. cable channels re-airing popular broadcast programs -- known in the industry as repurposing -- has grown in popularity in recent years, as program makers look for new outlets in an effort to recoup more of their costs.

But the reverse, where a broadcast network buys a program from a cable channel, has been relatively rare especially for repurposing programs to air in prime time.

In NBC's case, the network announced plans last month to air four episodes of "Forensic Files," a popular series on cable network Court TV, including two original episodes and two rebroadcasts of shows that debuted on cable.

NBC spokesman Curt King said the network has been so satisfied with "Forensic Files" it has exercised an option to broadcast a fifth episode on Sept. 14.

"We were very excited about the performance, very encouraged," King said. "It's in keeping with our goal of having programming that's original to NBC during the summer months. We'll continue with that goal."

In a similar deal, ABC said earlier this month it would rebroadcast four episodes of the popular USA Network series "Monk," a show about a neurotic detective.

Since then, ABC has aired three episodes of "Monk," while NBC has aired one episode of "Forensic Files," all with strong results.

"Monk," which plays in the difficult Tuesday evening spot opposite Fox's summer reality hit "American Idol: The Search for a Superstar," has seen its audience grow from 7.1 million viewers in its first airing to 9.2 million viewers this week.

It has finished second in its time period, behind only "Idol," in its last two outings.

Meanwhile, "Forensic Files" attracted 9.1 million viewers in its Sunday evening debut on NBC, finishing second for its time period.

Both shows have also done well among advertiser-coveted 18- to 49-year-old viewers.

"We're thrilled with 'Monk's' performance," said ABC spokesman Kevin Brockman. "Obviously, we went into this knowing it was an innovative summer experiment. We couldn't be more pleased with how the show has performed in an incredibly difficult time period facing very stiff competition."

Brockman said ABC has the right to rebroadcast any of the remaining episodes of "Monk" beyond the original four it ordered, but he had no comment on the network's future plans.

A network source said it is not out of the realm of possibility that the show, given its low cost, could pop up again later in ABC's new season in a time period that would otherwise be difficult to fill.

Meanwhile, the rival Fox network has no immediate plans to buy any cable network series for broadcast in prime time, said spokesman Scott Grogin.

"We're very happy with our development," he said.

ABC is a unit of the Walt Disney Co., NBC is a unit of General Electric Co. and Fox is a unit of News Corp. Ltd. The fourth major broadcast network is Viacom Inc.unit, CBS.