This paper investigates subjective, behavioral and neurophysiological changes due to treatment with oral sustained-release morphine in six patients with severe non-malignant pain. Patients rated their mood and clinical pain on visual analog scales (VAS). Experimental pain reactions were quantified by ratings on categorical scales and evoked cerebral potentials (LEP) in response to standardized laser stimuli. A standard auditory oddball task provided reaction time (RT), errors, N1 and P2 of late auditory evoked potentials (AEP), and a P300 component. It was used to measure vigilance and cognitive performance. In parallel with clinical pain reduction, laser pain ratings and LEP amplitudes were significantly reduced. In contrast, auditory P2 and P300 amplitude were found to be even enlarged under morphine. RT and mood also failed to indicate any sedation. It is concluded that LEP indicated the analgesic morphine effects whereas late potentials and P300 from auditory stimuli reflected the perceptual-cognitive status which, instead of being deteriorated by morphine-induced sedation, improved probably due to the removal of pain as a mental stressor.