Physical dependence on physiologically released endogenous opiates.

Mice which had been submitted to a chronic schedule of warm water swimming exhibited a naloxone precipitated withdrawal behaviour which was remarkably similar to that produced in mice following chronic morphine treatment. These results are consistent with the activation of endogenous opiates during swim stress in mice and present the possibility that opiate receptors are activated in a manner analogous to the repeated application of exogenous opiates, producing both tolerance and withdrawal-like behaviour.