Newborn rats were adrenalectomized at 0 h after birth and the development of enzyme catechol- o-methyltransferase (COMT) in heart, lung, liver and kidney at 3, 5, 10, 15 and 20 days of postnatal life was studied. Heart of adrenalectomized young rats showed an increase of statistical significance from the value of normal rats after 10 days of the ablation of the adrenal gland. During 15 and 20 days of postnatal life cardiac COMT activity remained lower than the activity of normal young rats. COMT in the lung of adrenalectomized rats was lower than the normal rats after 3, 5, 10 and 15 days postoperatively. The kidney also showed decrease in activity of COMT during 10 and 15 days of postnatal life from control values but at 20 days, there was no difference between the two groups. Liver COMT remained higher than the controls at 3, 5 and 10 days after adrenalectomy with a return to the level of normal animals afterwards. The results suggest that the evolution of enzyme COMT during postnatal life is somehow dependent on the activity of the adrenal cortex since adrenal demedullation did not produce any marked and significant effect on COMT activity in all the 4 organs. After 20 days of adrenal demedullation enzyme activity in the heart and lung was slightly higher than controls but these increases were without any statistical significance.