Plasma corticosterone and DNA synthesis were measured during 2 circadian periodes following the 25th day of rats. Corticosterone (5 mg/kg) injected at 05 h (1 h before the normal corticosterone bathyphase) inhibits and dissynchronizes the nycthemeral evolution of the two parameters during the two subsequent periods. The same corticosterone administration injected at 17 h (1 h before the normal corticosterone acrophase) inhibits the first DNA synthesis wave but both parameters are nycthemerally restored from the second period. In this last case, the area under the second DNA curve compensates the inhibition of the first wave. The results are discussed in the view of chronocorticotherapy recommended in patients.