The effect of dietary restriction (DR) on cell proliferation determined by cell cycle analysis in tissues of young and old mice was investigated. Using the percentage of S-phase cells as an index of cell proliferation, we found that DR inhibited cell proliferation in spleen and thymus in young mice. No significant changes were found in bone marrow and kidney in the ad libitum (AL) or DR mice regardless of age. In old mice, the DR effect was observed in spleen only. When age increased, a parallel decline in cell proliferation was evidenced by a reduced % of S-phase cells. DR produces a greater cell cycle effect in the young mice than in the old mice. The present data suggests that inhibition of cell proliferation by DR may be affected by type of tissue, age, length of DR, and capacity or rate of cell proliferation.