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Ten healthy young male volunteers received in random sequence 10 mg of morphine sulfate intravenously and by lumbar epidural route during two 26-hour study sessions, in order to observe the appearance and resolution of the following side effects: (a) pruritus, (b) nausea, (c) vomiting, (d) urinary dysfunction. With the exception of one subject, who experienced transient (2 hours) nausea, none of the subjects experienced any adverse side effects after the intravenous morphine. However, all subjects experienced some degree of one or more complications, starting 3 hours after the epidural administration: generalized pruritus started at 3.0 ± 0.3 hours (nine of 10 subjects, mean ± SD) and lasted 5.3 ± 4.0 hours. Nausea occurred in six subjects at 4.0 ± 0.6 hours, and lasted for 3.0 ± 2.1 hours; vomiting occurred at 6.3 ± 2.0 hours in five of the nauseated subjects. Urinary retention of varying intensity and duration appeared in nine subjects and required pharmacologic intervention in six subjects. Serum levels of unmodified morphine were measured at various times after administration during both sessions and did not correlate with the incidence or temporal appearance of side effects. Serial evaluation of dermatomal level of hypalgesia to ice and pin scratch demonstrated a progressive spread in the rostral direction after epidural morphine; trigeminal areas were affected by 9 hours in five of the 10 subjects. The stereotyped sequence of side effects after 10 mg of morphine by the epidural route can be interpreted to reflect widespread dispersion of morphine throughout the subarachnoid and ventricular cerebrospinal fluid.