Angiotensin II (ANG II) type 1 receptors are found in the mouse suprachiasmatic nucleus (SCN), the site of the circadian pacemaker, but their significance for circadian timekeeping is unknown. We examined circadian rhythms of wheel running and drinking in angiotensin AT(1a) receptor knockout (KO) mice. Mean daily running and drinking activity were elevated in KO mice under a light-dark (LD) cycle and in constant dark (DD). These increases were confined to the usual active (dark) period, thus, the 'amplitude' of running and drinking rhythms was higher in KO mice. The phase of entrainment to LD (measured by the onset of the daily active period) did not differ between groups, either in LD or on the first day of DD ('unmasked' phase). KO mice showed a modestly shorter free-running period (tau) in DD. The direction and magnitude of phase shifts to light pulses at two circadian times (CTs) in DD did not differ between groups. Core functions of the circadian system appear intact following AT(1a) receptor KO. The modestly shorter tau and increased rhythm amplitude in KO mice may be secondary to an effect of the mutation on the level of running and drinking activity.