The objective of the present study was to evaluate the effects of EMD 61753 (asimadoline), a kappa-opioid receptor agonist with restricted access to the central nervous system, on postoperative pain in patients who underwent knee surgery and on nociceptive thresholds and inflammation in rats treated with Freund's complete adjuvant. Patients treated with EMD 61753 (10 mg p.o.) tended to report an increase in pain, as evaluated by a visual analog scale and by the time to the first request for and the total amount of supplemental analgesic medication. The global tolerability of EMD 61753 was assessed as significantly inferior to that of a placebo by the investigator. In rats, the bilateral intraplantar (i.pl.) injection of EMD 61753 (0.1-3.2 mg) resulted in dose-dependent antinociception in both inflamed and noninflamed paws, with a peak at 5 min after injection, as evaluated by the paw pressure method. However, at later time points (1 h-4 days), a significant decrease in the paw pressure threshold was observed, confirming its tendency toward a hyperalgesic action in humans. This was accompanied by an increase in paw volume and paw temperature, with a peak at 6 h after injection. EMD 61753 (1.6 mg)-induced analgesia was blocked by the peripheral opioid receptor antagonist naloxone methiodide (2.5-10 mg/kg s.c.) and by the kappa receptor antagonist nor-binaltorphimine (0.1 mg; i.pl.). In contrast, EMD 61753 (1.6 mg)-induced hyperalgesia and increases in paw volume and paw temperature were blocked neither by naloxone methiodide (10-40 mg/kg s.c.) nor by dizocilpine maleate (0.003-0.009 mg i.pl.), a N-methyl-D-aspartic acid receptor antagonist. These data show differentially mediated peripheral actions of EMD 61753: kappa-opioid receptor-induced analgesia and nonopioid, non-N-methyl-D-aspartic acid hyperalgesic and proinflammatory effects.