Gabapentin and S-(+)-3-isobutylgaba are anticonvulsant agents that selectively interact with the alpha2delta subunit of voltage-dependent calcium channels. This report describes the activities of these two compounds in a rat model of postoperative pain. An incision of the plantaris muscle of a hind paw induced thermal hyperalgesia and tactile allodynia lasting at least 3 days. Postoperative testing was carried out using the plantar test for thermal hyperalgesia and von Frey hairs for tactile allodynia. A single s.c. dose of gabapentin, 1 h before surgery, dose-dependently (3-30 mg/kg) blocked the development of allodynia and hyperalgesia with a minimum effective dose (MED) of 10 and 30 mg/kg, respectively. The highest dose of gabapentin prevented development of hyperalgesia and allodynia for 24 and 49 h, respectively. Similar administration of S-(+)-3-isobutylgaba also dose-dependently (3-30 mg/kg, s.c.) prevented development of hyperalgesia and allodynia with MED of 3 and 10 mg/kg, respectively. The highest dose of S-(+)-3-isobutylgaba completely blocked development of both nociceptive responses for 3 days. The administration of S-(+)-3-isobutylgaba (30 mg/kg s.c.) 1 h after surgery also completely blocked the maintenance of hyperalgesia and allodynia, but its duration of action was much shorter (3 h). The administration of morphine (1-6 mg/kg s.c.) 0.5 h before surgery prevented the development of thermal hyperalgesia with a MED of 1 mg/kg. However, unlike gabapentin and S-(+)-3-isobutylgaba, it had little effect on the development of tactile allodynia. It is suggested that gabapentin and S-(+)-3-isobutylgaba may be effective in the treatment of postoperative pain.