1. The present work describes the catechol-O-methyltransferase (COMT) activities in the liver and kidney of developing and adult rats (aged 3, 6, 9, 18, 30 and 60 days; n = 5 per group) and evaluates the enzyme sensitivity to inhibition by tolcapone, a reversible COMT inhibitor. 2. COMT activity, evaluated by the ability to methylate adrenaline to metanephrine, was determined in liver and kidney homogenates prepared in 0.5 mM phosphate buffer (pH = 7.8) containing pargyline (0.1 mM), MgCl2 (0.1 mM), EGTA (1 mM) and S-adenosyl-L-methionine (0.1 mM). Vmax (in nmol mg-1 protein h-1) of liver COMT was found to decrease gradually with age, from 5.3 +/- 0.5 at the age of 3 days up to 2.9 +/- 0.2 at the age of 60 days; for the same age range, Km values (in microM; geometric means with 95% confidence limits) increased from 3.3 (1.0, 7.5) up to 13.1 (2.1, 24.1). At the age of 3 days, Vmax values for kidney COMT (2.6 +/- 0.1) were lower than those for the liver COMT. However, Vmax values for kidney COMT were found to increase up to 6.2 +/- 0.6 at the age of 18 days and then declined by 44% at the age of 30 and 60 days. In kidney, aging was also accompanied by an increase in Km values for COMT (from 2.7 [1.1, 4.3] up to 24.0 [11.7, 36.3]). 3. The sensitivity of liver and renal COMT activity to tolcapone was markedly dependent on the age, 3-days old rats being more sensitive to tolcapone than older animals. The IC50 values (in nM) for inhibition of liver COMT by tolcapone increased gradually with age, from 41 (26, 65) at the age of 3 days up to 720 (640, 800) at the age of 60 days. As was found in the liver, IC50 values (in nM) for inhibition of kidney COMT by tolcapone also increased with age, from 8 (6, 10) at the age of 3 days up to 177 (131, 240) at the age of 60 days. In all experimental groups, the IC50 values for inhibition of liver COMT by tolcapone was higher than those for kidney COMT. 4. In conclusion, these results suggest that aging is accompanied by a decrease in liver and kidney COMT affinity for the substrate (evidenced by the increase in Km values) and a decrease in sensitivity towards inhibition by tolcapone (evidenced by the increase in IC50 values). Furthermore, kidney COMT is shown to be more sensitive to inhibition by tolcapone than liver COMT, irrespective of the age of the animal.