Antipeptide antibodies against the kappa-opioid receptor were used to test whether acute or chronic exposure to kappa agonists altered the phosphorylation state of the kappa-opioid receptor. Immunoprecipitation of the kappa receptor from guinea pig hippocampal slices preincubated in [32P]orthophosphoric acid revealed a basal phosphorylation of the kappa-opioid receptor. The amount of 32P incorporation into the receptor was increased following a 75-min treatment with the kappa agonist U50,488H. This effect was blocked by the selective kappa receptor antagonist norbinaltorphimine. The time course of this change in the phosphorylation state of the receptor correlated with a desensitization of the electrophysiological response to kappa agonists measured in the dentate gyrus of hippocampal slices. The phosphorylation state of the kappa-opioid receptor was also elevated in brain slices from guinea pigs made tolerant to U50,488H by 5 days of continuous exposure and then maintained in kappa agonist to avoid acute opiate withdrawal. The results of this study show that the kappa-opioid receptor was phosphorylated in an agonist-dependent manner in brain slices taken from untreated and U50,488H-tolerant animals.