Isoprenaline markedly relaxed rat uterine muscle and increased its content of cyclic AMP. The cyclic AMP concentration reached its highest value, about ten times the control after 3 min., after which it declined rather quickly at first but later more slowly. After 60 min. the cyclic AMP content reached twice the prestimulation value. The muscle remained relaxed all the time. If the incubation medium from the uterus treated with isoprenaline for 60 min. was added to a fresh muscle, a normal response was elicited. Addition of a new dose of isoprenaline to a muscle previously incubated with isoprenaline for 60 min. only gave a minimal rise in cyclic AMP after 3 min. The presence of catechol or corticosterone, which both decreased the inactivation of isoprenaline, showed no effect on the decrease of the isoprenaline-stimulated cyclic AMP level. Nor could the decrease in the cyclic AMP response be prevented in the presence of GTP,F-, adenosine or in Ca2+-deficient medium. The addition of theophylline and puromycin both lead to a slower decline of cyclic AMP content, but after 60 min. the value returned to near that seen ater only isoprenaline. Our studies on isoprenaline-induced desensitization of rat uterus have led us to the conclusion that the change in the capacity of the preparations to accumulate cyclic AMP may be a result of either changes in the beta-adrenoceptors or in the cyclase-system.