Contents of immunoreactive β-endorphin and maximum of ³H-morphine binding was measured in the rat midbrain homogenates from different subgroups at 24 hour interval over 24 hours. Animals were adapted to the light-dark cycle(L : D, 12: 12) or constant darkness (D : D, 12 : 12) for 3 weeks. After the adaptation, 0.5ml of physiologic saline or drug was administered twice a day for 2 weeks. A highly significant circadian rhythm with the peak(94.8 ± 7.7 fmole/mg protein) at 06:00 and the nadir (27.6 ± 2.4 fmole/mg protein) at 18:00 was observed in constant of group. Constant dark or treatment of reserpine, pargyline, imipramine, amphetamine and chlorpromazine modified the diurnal rhythm in the time of peak and nadir, shape, phase amplitude and 24 hour mean of β-endorphin contents. Opiate receptor binding by ³H-morphine also showed highly significant diurnal change in control and constant dark adapted rats. Statistical analysis by one-way analysis of variance and two-way analysis of variance indicates that the¡¤re are highly significant differences between the diurnal change of β-endorphin in control and those constant dark adapted and drug treated groups. However diurnal change of Maximum ³H-morphine binding is closely related to the change of β-endorphin contents. The results are interpreted with regard to the circadian rhythm of beta-endorphin contents, its modification by psychoactive drugs and possible mechanism of diurnal change of opiate receptor in brain.