Using an Alzet osmotic minipump, we examined the effect of continuous infusion of O-conotoxin, a N-type voltage-sensitive calcium channel blocker, into the third cerebral ventricle above the suprachiasmatic nucleus (SCN) of the hypothalamus on the circadian drinking rhythm of rats maintained under a 12-h light and 12-h dark cycle and constant darkness. O-Conotoxin (10-6 mol/h) infusion disrupted the rhythm in both conditions. In contrast, infusions of nifedipine (104 mol/h), a dihydropyridine sensitive L-type calcium channel blocker, did not eliminate the rhythm. These findings suggest that N-type voltage-sensitive calcium channels are involved in the mechanism of generation of the circadian rhythm driven by the circadian oscillator in the SCN.