After a latency period the catechol- O-methyltransferase inhibitor tropolone blocked increasingly the contractile response of rabbit isolated pulmonary artery elicited by electrical-field stimulation. Furthermore, tropolone contracted the artery. These inhibition effects were irreversible. In contrast, phentolamine caused a reversible equilibrium block with a rapid onset. Tropolone, which had been subjected to oxygenated physiological salt solution at 37 °C for a prolonged period, accelerated the development of neurogenic block compared to a freshly prepared solution of tropolone. This suggests that the blocking effect may in part be due to a break-down product of tropolone. Tropolone did not alter the electrical-field stimulation induced outflow of tritium from the pulmonary artery preloaded with <sup>3</sup>H-noradrenaline. Initially, tropolone did not alter the cumulative concentration-contraction response curves of exogenous noradrenaline, acetylcholine and serotonin. However, after prolonged exposure of the pulmonary artery to tropolone, this agent inhibited the response elicited by these agonists in an apparent noncompetitive manner. The results suggest that tropolone exerts a direct blocking effect on vascular smooth muscle, which probably is neither mediated through adrenergic receptors nor due to its catechol- O-methyltransferase inhibitory activity. The observed non-specific effects of tropolone must be taken into account when tropolone is employed as an enzyme inhibitor.