1. This work examines the effect of L-tyrosine on the antinociceptive and non-nociceptive actions of morphine in mice. The antinociceptive action was measured using the hot plate method, and the non-nociceptive actions included the effect on concentrations of glucose, cholesterol and triglycerides in serum or blood, and the effect on gastrointestinal transit time (GITT) and rectal temperature. 2. L-tyrosine (25, 50, 100 and 200 mg/kg), subcutaneously (s.c.) dose-dependently potentiated the antinociceptive action of morphine (5 mg/kg, s.c.). L-tyrosine alone did not produce significant antinociceptive action, nor did it affect any of the other non-nociceptive actions measured. 3. Acute administration of morphine (5, 10 and 20 mg/kg) produced dose-dependent increases in blood glucose concentration which were insignificantly potentiated by L-tyrosine (25-200 mg/kg) when given together with morphine. Morphine produced dose-dependent and significant decrease in serum triglycerides concentrations, an effect which was not influenced by L-tyrosine treatment. Serum cholesterol was not affected by treatment with morphine, either alone or when given with L-tyrosine. 4. Morphine produced dose-dependent and significant decreases in GITT which was not affected with L-tyrosine.