This study compared the subjective, physiological and psychomotor effects of a novel muscarinic analgesic (LY297802) and oral morphine in healthy volunteers. Nine, non-dependent, occasional drug users participated in nine experimental sessions in which they received the following conditions: placebo, 0.1, 0.3, 0.56 and 1 mg of oral LY297802 and 10, 30, 56 and 100 mg of oral morphine. Subjective drug effects were assessed using the Visual Analog Scale (VAS), the Addiction Research Center Inventory (ARCI) and subjective and objective agonist and antagonist scales of the Adjective Rating Scale (ARS). These measures were collected 30 min before and every 30 min post drug administration for a 4-h period. Psychomotor performance was evaluated using the Digit Symbol Substitution Test (DSST) at these same time intervals. Physiological measures were collected continuously throughout the sessions. Oral morphine produced significant increases in some subjective effects scales, including elevations on the VAS, ARCI and ARS. In contrast, LY297802 did not engender changes different from placebo on any of these indices. Morphine produced significant dose-dependent effects in DSST performance, heart rate, blood oxygen saturation and pupil diameter. LY297802 significantly and dose dependently increased heart rate, mean arterial pressure and diastolic blood pressure. These results suggest that LY297802 does not induce subjective effects similar to morphine, but that it has some significant physiological effects.