The evidence from a variety of studies suggests the involvement of serotonin (5-HT) in both the response of the suprachiasmatic nuclei neurons to light and the phase-resetting properties of nonphotic stimuli. Although it is well known that 5-HT agonist causes a phase shift of locomotor activity rhythm, there are few reports of experiments on the infl uence of serotonin upon the locomotor activity rhythm of nocturnal rodents kept under constant light (LL) conditions. In our previous experiments we characterized the phase response curve (PRC) for (R)-(+)-8-OH-DPAT in LL in mice and we discovered that the shape of this PRC is similar to the PRC for dark pulses under LL. It suggests that mechanism response for phase shifts of locomotor activity rhythm induced by 5-HT1A agonist and by dark pulses may be the same. Therefore, we have examined the effect of (R)-(+)-8-OH-DPAT on the phase shifts of locomotor activity rhythm induced by dark pulses. Eight week old C57BL/10 male mice were kept in a soundproof room in individual cages with free access to a running wheel. The wheel-running activity rhythm was recorded under 12:12 LD and under LL. In the LL (R)-(+)-8-OHDPAT was i.p injected every two hours during twenty-four hours. Injections were given at 30 minutes before 4 hours dark pulses. The analysis of the phase shifts of the locomotor activity rhythm indicated that at some time points (R)-(+)-8-OH-DPAT increases dark pulses phase-shifts whereas in other points decreases were observed.