1. Highly localized microinjections of serotonin into the nucleus tractus solitarius in rats produced dose-dependent hypotension and bradycardia and electroencephalogram (EEG) changes consistent with an alerting response.
2. Microinjection of the serotinin antagonist metergoline blocked the cardiovascular and EEG effects of serotonin but did not affect the same responses elicited by the microinjection of l-glutamate.
3. Microinjections of l-glutamate produced similar effects; acetylcholine, which lowered arterial pressure and heart rate, did not affect the EEG.
4. Bilateral injections of metergoline into the nucleus tractus solitarius occasionally produced a rise in arterial pressure but did not affect the baroreceptor reflex.
5. Serotonin may integrate cardiovascular and behavioural responses mediated by the nucleus tractus solitarius.
6. Neuronal populations within the nucleus tractus solitarius may be neurochemically and functionally differentiated.