It has been shown that some cytochrome P450-dependent enzyme activities could present daily fluctuations, particularly CYP3A isoenzymes which are enhanced during the dark period. The aim of this study was to investigate whether age and photoperiodic conditions at different circadian stages could influence these fluctuations. Young mature (10 weeks) and old (22 months) Wistar rats were initially exposed to light-dark cycles 12:12 during 4 weeks, and secondly 18:6 for either one week or six weeks. Erythromycin N-demethylase (CYP3A-dependent), 7-ethoxycoumarin O-deethylase (CYP1A-dependent) and aniline 4-hydroxylase (CYP2E-dependent) activities were determined in liver and kidney microsomes at different hours after darkness onset (HADO). In addition, liver and kidney GSH, GSHPx, ATP, TBARS were determined. During the LD 12:12 cycle, while no significant modification was observed in CYP1A- and 2E-dependent enzyme activities as functions of HADO, erythromycin N-demethylase activity (CYP3A-dependent) showed a significant increase during the second third of the dark period in both young and old rats. After switching to a LD 18:6 cycle, this variation was still observed during second third of the dark period, to a lesser but still significant degree, with no difference between one week and six weeks exposure to the new photoperiod. It can be noted that the old rats showed a significantly lower level of erythromycin N-demethylase activity than the young rats, in parallel to a decrease in GSH, GSHPx and ATP, and an increase in TBARS. These results confirm the lower resistance of old animals to oxidative stress. The observed variations in metabolism parameters underline the need for study designs in pharmaco-toxicology taking into account the possible risks induced by circadian changes, especially in aged subjects.