Abstract The effects of social crowding in rats on nociception and analgesic actions of μ- and k-opioid agonists against qualitatively different stimuli were examined. Crowding produced higher control thresholds for electroshock and paw pinch pressure but not for noxious heat. Naloxone did not effect shock thresholds in crowded or noncrowded rats but it produced hyperalgesia in both groups when pressure and heat were the stimuli. Crowding reduced the antinociceptive potency against pressure of k- and μ-agonists but the opposite effect was seen with heat. A comparison of the ratios of potencies against pressure and heat indicates that crowding differentially modified k- and μ-receptor systems.