A biotelemetric procedure was used to investigate the thermic and kinetic responses of rats to a range of doses (0.0-30.0 mg/kg) of morphine sulphate. Both responses demonstrated a biphasic dose-response pattern, with depressant effects (hypothermia and hypokinesis) predominant at high doses, and excitatory effects (hyperthermia and hyperkinesis) predominant at low doses. The time-courses of the thermic and kinetic responses were found to be uncorrelated. With repeated doses of morphine, the depressant effects were attenuated (tolerance) while the excitatory effects were augmented (sensitization). These results are interpreted in the light of current views of opiate function. The value of biotelemetry as a means of avoiding the confounding effects of drug-induced thermic responses and thermic responses induced by stressful temperature-assessment procedures is discussed.