Basal metabolic rate, basal colonic temperature, and response to intraperitoneal injection of tryamine were examined in three age groups: young (3 months), adult (12 months), and old (24 months) of male Fischer 344 rats. Basal metabolic rate of sedentary male F344 rats decreased significantly between 3 months (21.5 ml O2 X kg-1 X min-1) and 1 year (19.4 ml) of age and continued to decrease in the oldest animals (19.0 ml). The decrease between 1 and 2 years was not statistically significant. Basal colonic temperature showed a step-wise significant increase with age. Mean colonic temperature was 36.7 degrees C at 3 months, 37 degrees C at 1 year, and 37.5 degrees C at 2 years. Higher optimal doses of tyramine were required for 1- and 2-year-olds than for 3-month-olds. The optimal dose was 2 mg X kg-1 for 3-month-olds and 5 mg X kg-1 for 1- and 2-year-olds. Although the optimal dosages were the same for the 1- and 2-year-olds, 2-year-olds had lower peak response and the response was slower than for the 1-year-olds.