Somatosensory evoked potentials (SEP) at present seem to be the most reliable and accurate method for the investigation of compounds modulating pain sensation. Besides detecting changes in the duration and the potency of action of analgesics, their possible site of action in the sensory pathways may be derived by this method. In ten trials using two trained canines the efficacy and the duration of action of a new and short acting opioid, alfentanil was evaluated on SEP using cumulative doses (3, 30, 60, 120 micrograms/kg). Concomitantly arterial blood gases were analyzed for possible respiratory depressant effects. The results demonstrate that alfentanil induces a short lasting suppression of the N100 peak as well as a latency change of the N140 peak to N220, the latter lasting for more than 80 min. These effects saturate at doses higher than 30 micrograms/kg. The significant change in arterial blood gases - a fall in PaO2 and an increase in PaCO2 - are most prominent with 30 micrograms/kg and 120 micrograms/kg, respectively. It is of short duration and correlates with the suppression of the N100 peak. The latency changes seem to correlate with a marked sedatory effect. Due to the differential effects induced by the opioid, an interaction with different opioid subreceptor sites is postulated.