Although the effect of systemic opioids on somatosensory evoked potentials has been well described, little is known about the interaction between intrathecally administered opioid analgesics and somatosensory evoked potentials. Accordingly, the influence of intrathecally administered morphine on posterior tibial nerve somatosensory cortical evoked potentials (PTSCEPs) was investigated in 22 unpremedicated, awake, neurologically normal patients scheduled to undergo elective abdominal or pelvic procedures. Patients were randomly assigned to receive either preservation-free intrathecal morphine sulfate (ITMS) or placebo. After baseline PTSCEP, heart rate and, mean blood pressure were recorded, ITMS (15 micrograms.kg-1) was injected via standard dural puncture with the patient in the lateral position. PTSCEPs, heart rate, and mean blood pressure were recorded again at 5, 10, 20, 30, 60, 90, and 120 min. Control patients were treated identically (including position, sterile preparation, and subcutaneous tissue infiltration with local anesthetic), except for lumbar puncture, and were unaware of their randomization. Before administration of ITMS, PTSCEP P1, N1, P2, N2, and P3 latencies were 39.4 +/- 3.2, 47.6 +/- 3.9, 59.2 +/- 3.2, 70.4 +/- 3.7, and 84.6 +/- 5.5 ms, (mean +/- standard deviation), respectively. The corresponding P1-N1, N1-P2, and P2-N2 amplitudes were 2.4 +/- 1.1, 2.4 +/- 1.1, and 2.3 +/- 0.9 microV, respectively. There were no significant changes over time between the control and ITMS groups. PTSCEPs resulting from left-sided stimulation were not different from those elicited by right-sided stimulation. All ITMS patients had intense postoperative analgesia for at least 24 h. It is concluded that ITMS does not affect PTSCEP waveforms in the 35-90 ms latency range during the awake state.(ABSTRACT TRUNCATED AT 250 WORDS)