San Quentin Prison, for more than a century one of the toughest places in the nation to do time, has been emptied of virtually all its most hardened criminals.

Some 1,750 of them, more than half the population of the 137-year-old fortress that once held Charles Manson and Sirhan Sirhan, have been sent to new prisons throughout California as part of a $5 billion prison expansion.In their place are minimum- and medium-security prisoners, including many who live in trailers outside the prison walls on San Quentin's fenced-in grounds and have little contact with the higher-risk inmates inside.

California's Death Row remains here for the 252 prisoners, all men, facing execution by cyanide gas. But aside from them, "we have lost almost all of our hard-core (inmates)," said prison spokesman Cal White, not unhappily.

"That once housed the most desperate people in California," he said, pointing to an empty five-tier cell-block. "Now, it's just a temporary holding facility."

The concrete-and-steel compound ringed by guard towers on the edge of San Francisco Bay has long been depicted as one of America's toughest prisons.

Its inmates included Sirhan, who assassinated Robert F. Kennedy in 1968, and Caryl Chessman, convicted of Los Angeles' "red light bandit" sex attacks of 1948. He became a best-selling author behind bars before being executed in 1960.

But in the past two years, most of the highest-risk prisoners have been sent to three new prisons. Manson, the leader of a murderous cult and perhaps the most notorious prisoner in California, went to the new Corcoran prison near Fresno.

Corcoran is one of eight prisons opened in California since 1980. The state's prison population has tripled since then to more than 75,000, more than any other state. None of the older prisons has been affected more than San Quentin.

On the outside, the complex appears as forbidding as ever. Several buildings were erected in the middle to late 19th century - one has the date 1857 chiseled in its front - and even the more recent structures are decades old.

On the inside, what authorities call San Quentin's "change in mission" is quickly apparent. In addition to holding medium- and minimum-security inmates, it serves as a reception center, where new inmates are sent for orientation and classification.

"The stress level here has dropped considerably over the past couple of years," White said. "We used to have some kind of incident every day. It wasn't uncommon to have a `lockup' every month. Now that's pretty much of a rarity."

The prison has been extensively remodeled, including a $28 million overhaul of an old cellblock. In the hospital, 55 years old but newly refurbished, is the dental ward, a spic-and-span suite that smells powerfully of disinfectant.

"They call me `Dr. Painless,' " said Dr. Glenn Jamieson, a bouyant, breezy man who has taken care of prisoners' teeth here for 22 years. He calls the ward "the only air-conditioned place in the prison . . . as good as any on the outside."