There is some evidence in the literature that catecholamines relax uterine and other types of smooth muscle by increasing tissue levels of cyclic adenosine monophosphate (cyclic AMP). In the present study, isoproterenol completely relaxed uterine strips obtained from estrogen-primed rats and also increased tissue levels of cyclic AMP and phosphorylase a. In uterine strips depolarized and put into contracture for 15 minutes by 127 mM K+, isoproterenol did not increase phosphorylase a or cyclic AMP but was still capable of producing relaxation. When uterine strips were exposed to the methoxy derivative of verapamil, D-600, a compound known to prevent the influx of calcium, the uterus relaxed completely without an increase in cyclic AMP. The addition of isoproterenol at this stage resulted in an increase in cyclic AMP similar to that noted in nondepolarized uterine strips. The addition of 127 mM K+ also resulted in time-dependent biochemical changes as well as contracture. Cyclic AMP was increased 3-fold after 2 minutes of K+ depolarization and phosphorylase a was increased as well. The increase in cyclic AMP was prevented by propranolol but propranolol did not affect the contracture response to K+. D-600 prevented contracture but did not affect the K+-induced increase in cyclic AMP. The data suggest that an increase in whole tissue levels of cyclic AMP are not necessary in order for isoproterenol to relax depolarized rat uterine strips. The data also suggest that intracellular calcium levels can affect the level of cyclic AMP in the rat uterus.