The circadian rhythm of ethylmorphine N-demethylase (ND) and hexobarbital sleeping time (HBS) was studied in noninduced and dichlorophenoxy propionic acid (DP) induced adult male rats. DP was gavaged (40 mg/kg bw) at 1.00, 7.00, 13.00 or 19.00 hr. After 21 days the above mentioned parameters were determined corresponding to the time point of DP administration. ND showed a biphasic oscillation (peak activities at 1.00 and 13.00 hr), whereas HBS was significantly shortened only at 1.00 hr. The effects of DP--ND-induction and HBS-shortening--were significant only, when the circadian phase of these parameters was low (i.e. at 7.00 and 19.00 hr for ND and at 7.00 hr for HBS). Consequently, attention should be paid not only to circadian rhythms of the cytochrome P-450 system per se but also to circadian alterations of the inducibility of this system.