PGI2 1 or 10 micrograms iv caused a decrease of blood pressure and bradycardia in rats. PGI2 administered into lateral ventricle of rat brain [icv] exerted hypotensive effect and tachycardia. PGI2 10 micrograms icv increased histamine level in the hypothalamus and cortex of rat's brain, but it did not influence histamine content in the blood. Cimetidine [CMT] given 60 min earlier per os inhibited bradycardia caused by PGI2 iv administration and diminished hypotensive action and tachycardia produced by PGI2 icv.