Intraventricular injection of prostacyclin (PGI2) slightly depressed the general behavior and produced a weak hypothermia in rats. It shortened the response to a thermal nociceptive stimulus and intensified catalepsy caused by chloropromazine and haloperidol. PGI2 did not change concentration of noradrenaline, 4-hydroxytryptamine, 5-hydroxyindoleacetic acid and dopamine in different brain areas. It markedly lowered blood pressure and increased respiration. The duration of central hypotensive effect of PGI2 was shortened after 6-hydroxydopamine on 5,6-dihydroxytryptamine pretreatment. The possible involvement of a central mechanism in the hypotensive action of PGI2 requires further clarification.