The effects of the selective kappa agonist, U-50,488H, on the motor response to noxious mechanical stimulation when administered intrathecally, as well as on respiratory rate and PaCO2 when administered by intracerebroventricular injection, were compared with those of morphine in experiments on rats. The agonist U-50,488H caused a dose-dependent increase in the threshold to noxious stimulation but did not have a potential for depression of resting ventilation. The results suggest that selective kappa opioid agonists may be suitable for clinical spinal (epidural) analgesia without the associated risk of respiratory depression.