Profadol is a pyrrolidine derivative patented in the 1960s by pharmaceutical company Parke-Davis as opioid analgesic. Profadol acts as a mixed agonist-antagonist of the μ-opioid receptor and in preclinical studies, Profadol precipitates abstinence in morphine-dependent monkeys and can reverse pethidine- induced narcosis in nondependent monkeys. In morphine-dependent human subjects, Profadol was also found to pre¬cipitate acute abstinence syndromes, with a potency 40 to 50 times less than that of nalorphine. Profadol, unlike other morphine-antagonists, does not produce nalorphine-like subjective effects. Over a fourfold range of doses, this drug was found to produce subjective effects indistinguishable from those of morphine. Also unlike other morphine-antagonists, profadol is quite active on the "classical" rodent tests for analgesia. It is about 1.3 times as potent as pethidine on the mouse hot-plate test, and about four times as potent on the rat tail-pressure test.