In vivo studies have suggested that the κ opioid system can partially inhibit the development of physical dependence to μ agonists. Vice versa, activation of μ receptors may inhibit the expression of physical dependence to κ agonists. We studied μ-κ interactions in the isolated guinea-pig ileum (GPI). In the isolated GPI briefly exposed to μ or κ agonists the addition of the respective antagonists precipitated a withdrawal contracture. After a first withdrawal response, however, some tissues failed to exhibit subsequent μ or κ withdrawal contractures. A withdrawal contracture to the selective μ antagonist, cyprodime, after repeated exposures to a selective μ agonist, dermorphin, was restored by nor-binaltorphimine (BNI), a selective κ antagonist. Vice versa, after repeated exposures to the κ agonist, U-50,488H, cyprodime restored tissue responsiveness to BNI. Tissues repeatedly exposed to dermorphin and washed after each exposure contracted to the addition of BNI. Tissues repeatedly exposed to U-50,488H contracted on the addition of cyprodime. These findings strongly suggest that exogenous agonist-elicited stimulation of the μ (or κ) opioid system indirectly activates the endogenous κ (or μ) system. The indirectly-activated endogenous system inhibits the withdrawal response to the exogenously-stimulated opioid system. In isolated GPI the μ and κ opioid systems thus appear to interact, regulating each other.