In rats, neonatal administration of monosodium glutamate (MSG) causes serious damage in some hypothalamic and circumventricular areas. The resulting loss of appropriate neurons important for the regulation of blood pressure (BP) may modulate cardiovascular system receptivity in these animals. In the present study, the reactivity of the cardiovascular system to intravenous injection of alpha1-adrenergic receptor agonist phenylephrine (200 microg/kg/ml) and angiotensin II (500 ng/kg in 0.6 ml for 2 min) was investigated in adult rats which had been neonatally treated with MSG or vehicle. BP parameters measured directly in conscious cannulated rats were continuously registered using a computerized system. Under basal conditions, MSG-treated rats had slightly lower systolic, diastolic and mean BP with significant differences in pulse pressure (systolic - diastolic BP). In MSG-treated animals, the maximal increase of mean arterial BP after phenylephrine and the duration of BP elevation after both agents were significantly reduced. Slopes of the linear portion of baroreceptor function curves in control and MSG-treated rats did not differ significantly, indicating that baroreflex efficacy was unchanged. The results obtained by perfusion of the hindlimb vascular bed in situ showed that the pressure responses to increasing doses of noradrenaline in MSG-treated rats were reduced. These findings demonstrate that neonatal treatment of rats with MSG lowers the responsiveness of the cardiovascular system, particularly in response to alpha-adrenergic stimulation. It is suggested that the attenuation of cardiovascular reactivity in MSG-treated rats is, at least partly, caused by diminished vascular responsiveness.