At the Salk Institute in La Jolla, Calif., scientists are trying to get time to run backward.

Biological time, that is. In the first attempt to reverse aging by reprogramming the genome, they have rejuvenated the organs of mice and lengthened their life spans by 30 percent. The technique, which requires genetic engineering, cannot be applied directly to people, but the achievement points toward better understanding of human aging and the possibility of rejuvenating human tissues by other means.

The Salk team’s discovery, reported in the Thursday issue of the journal Cell, is “novel and exciting,” said Jan Vijg, an expert on aging at the Albert Einstein College of Medicine in New York.

Leonard Guarente, who studies the biology of aging at M.I.T., said, “This is huge,” citing the novelty of the finding and the opportunity it creates to slow down, if not reverse, aging. “It’s a pretty remarkable finding, and if it holds up it could be quite important in the history of aging research,” Dr. Guarente said.

The finding is based on the heterodox idea that aging is not irreversible and that an animal’s biological clock can in principle be wound back to a more youthful state.