Actions of quazodine (MJ1988) on smooth muscle.

1. Quazodine (MJ1988; 6,7-dimethoxy-4-ethyl-quinazoline) relaxed a variety of vascular and extravascular smooth muscle preparations and antagonized, non-competitively, several substances which contract smooth muscle.2. This activity was not due to beta-adrenoceptor stimulation or to alpha-adrenoceptor blockade.3. On the rabbit duodenum, the inhibitory effect was qualitatively similar to that of theophylline and was antagonized by the phosphodiesterase potentiator, imidazole.4. The activity of quazodine was similar to, but up to 18 times greater than, that of theophylline and it is suggested that at least part of its activity on vascular and extravascular smooth muscle is due to inhibition of phosphodiesterase.