The cardiovascular effects of microinjection of L-glutamic acid into the rostral ventrolateral medulla have been investigated in anaesthetized and in conscious, unrestrained rats. In conscious rats L-glutamic acid (0.36, 1.2 and 2.4 nmol) produced significant increases in blood pressure, generally accompanied by a bradycardia. A lower dose of L-glutamic acid (0.12 nmol) had no effect on blood pressure or heart rate. The cardiovascular responses elicited by L-glutamic acid (2.4 nmol) were completely abolished by intravenous infusion of the ganglion blocker, pentolinium. However, in urethane-anaesthetized rats, the pressor response (+27 +/- 2/+21 +/- 2 mmHg) to microinjection of L-glutamic acid (2.4 nmol) was markedly less than that seen in conscious rats (+64 +/- 6/+47 +/- 3 mmHg) and there was a tachycardia (+21 +/- 2 beats/min) rather than a bradycardia. These results corroborate previous studies, carried out in anaesthetized animals, indicating that stimulation of the rostral ventrolateral medulla provides direct and/or indirect excitatory drive to the preganglionic sympathetic neurons of the spinal cord to increase blood pressure, but demonstrate a marked difference in sensitivity in conscious vs urethane-anaesthetized animals.