Abstract Circadian rhythms of locomotor activity and plasma corticosterone were determined simultaneously in individual rats which were injected intraventricularly with 6-hydroxydopamine (6-OHDA), a specific depletor of catecholamines. In light-dark cycles (LD), 6-OHDA-treated rats showed essentially normal circadian rhythms of both functions. In 200 lux continuous light (LL) the locomotor activity in the drug-treated rats ran freely with a period slightly longer than 24 h: free-running parameters (mesor, amplitude and period) of the locomotor activity were the same as those of control rats. The circadian rhythm of plasma corticosterone in control rats also ran freely in LL, resulting in a reversal of their phase after 12 days exposure to LL. However, the rhythm disappeared in 6-OHDA-treated rats under LL: instead in these rats 3–4 episodes of secretion were observed over a 24 h period. These results suggest a difference in the biochemical background between the regulatory mechanism of circadian rhythm of locomotor activity and that of plasma corticosterone. It is also apparent that brain catecholaminergic components are indispensible for the circadian rhythm of plasma corticosterone to freerun normally in LL, but are dispensible in LD. A plausible model consisting of two oscillators is proposed to explain these findings.