We previously reported a marked enhancement of nocturnal sleep in pregnant and pseudopregnant rats and suggested a possible involvement of prolactin (PRL) in the modulation of sleep during pregnancy. In the present study, the release of PRL was blocked by a dopamine agonist, bromocriptine (CB-154), and time-course changes in sleep were analyzed in pregnant rats. After pregnancy was induced by fertile mating with males (Day 1), rats (n = 5) were injected with 1 mg of CB-154 subcutaneously 3 h before the onset of darkness for a 3-day period (Days 1–3 or 6–8). Electroencephalogram and electromyo-gram for evaluating the state of sleep and wakefulness were monitored continuously up to the reappearance of an estrous cycle in the treated rats. The CB-154 treatment resulted in abortion in all rats and suppressed the pregnancy-associated nocturnal increase in non-rapid-eye-movement sleep (NREMS) and rapid-eye-movement sleep (REMS). The CB-154 treatment also decreased the diurnal amount of NREMS significantly. The sleep-suppressing effect of CB-154 on Days 6-8 was induced more promptly than that on Days 1-3. In the night of proestrus following the termination of pregnancy (Day 4 or Day 12), both NREMS and REMS decreased to the baseline level observed at prepregnant proestrus. Thus, our results indicated that CB-154 neutralized the pregnancy-associated sleep changes, which might be due to the inhibition of the PRL release. It is suggested that PRL is at least partially responsible for the regulation of sleep during pregnancy.