The responses of systemic arterial blood pressure and heart rate to intra-cerebral injections of the excitatory neurotoxin, kainic acid, were examined in urethane-anesthetized rats. Injections of kainic acid into the antero-dorsal medial hypothalamus produced dose-related increases in both blood pressure and heart rate over a range of 30-1000 ng. Exophthalmos, mydriasis, increased respiratory rate and movements of the vibrissae were also noted. Injections of 1000 ng of kainic acid into the antero-dorsal medial hypothalamus produced significantly greater increases in blood pressure and heart rate than comparable injections of equimolar doses of the excitatory neurotoxins, quisqualic acid, N-methyl-D-aspartic acid or quinolinic acid. No differences in the magnitude of the cardiovascular responses to 1000 ng of kainic acid were detected between injections directed towards the paraventricular nucleus of the hypothalamus (PVN), lateral hypothalamus or lateral cerebral ventricle. In contrast, at doses of 30 ng, injections directed towards the paraventricular nucleus produced significantly greater responses than comparable injections into the lateral hypothalamus, medial thalamus or lateral cerebral ventricle. The distribution of radiolabelled kainic acid at this dose was found to extend ipsilaterally in the medial hypothalamus as far as 1 mm rostral and caudal to the injection site. The results suggest that excitation of neuronal cell bodies within the medial hypothalamus by excitatory neurotoxins produces acute increases in blood pressure and heart rate. However, widespread diffusion of kainic acid, in particular, was documented and caution in interpretation of the results produced by local intra-cerebral injections of this agent is recommended.