The opioid receptors μ, δ and κ have recently been cloned. Here we show that κ-agonists inhibit adenylyl cyclase activity in Chinese hamster ovary cells stably transfected with rat κ-opioid receptor cDNA. Chronic exposure of the cells to κ-agonists did not lead to significant desensitization of the capacity of the agonists to inhibit adenylyl cyclase. On the other hand, withdrawal of the agonist following the chronic treatment led to the phenomenon of supersensitivity (‘overshoot’) of adenylyl cyclase activity. Both the inhibition of adenylyl cyclase activity by the acute opioid treatment and the chronic agonist-induced supersensitivity are pertussis toxin sensitive, demonstrating involvement of Gi/Go proteins in both processes.