There are conflicting reports on the blood pressure effects of tyrosine. The aim of this study was to establish complete dose-response relationships and to compare the effects of various modes of administration of L-tyrosine in anaesthetised normotensive and spontaneously hypertensive rats. The intravenous injection of L-tyrosine, 0.2-0.4 mmol/kg, produced tachycardic and hypertensive effects in both species. The higher doses (0.8-1.6 mmol/kg) produced marked bradycardiac and hypotensive responses. Intracerebroventricular administration of L-tyrosine, 0.005-0.1 mmol/kg, had no statistically significant effects. Chronic dietary administration of L-tyrosine at the approximate daily doses of 0.7-55 mmol/kg was also without any significant effects. These results suggest that the controversies in the earlier studies could be due mainly to differences in doses and modes of administration. Our results also suggest that the cardiovascular effects of tyrosine are peripheral rather than central in origin although a central site of action cannot be excluded.