Histamine (HA) was given intracerebroventricularly (icv) to rats anaesthetized with pentobarbital. During the first minute after administration, HA alone elicited a small fall in blood pressure (BP) and a decrease in heart rate (HR), followed by a distinct increase in those parameters. Pretreatment with naloxone (Nx) in a dose of 0.1 microgram icv significantly decreased the rise in BP and augmented the increase in HR. When the rats were premedicated with atropine methyl bromide (ATMB), 15 min before histamine, higher doses of ATMB (100 micrograms icv or 25 micrograms ip) significantly inhibited the histamine induced BP rise. However, a very small dose (0.1 microgram) of ATMB stimulated the hypertensive response, but only after its peripheral and not central application. These changes were significantly reversed by icv pretreatment with Nx. HA-induced changes in HR, in contrast to BP, were not significantly modified by ATMB.