To examine the participation of endogenous cylooxygenase (COX) in the mnemonic effect of NC-1900, an arginine-vasopressin fragment analog. the latencies of mice in the step-through passive avoidance (PA) task were determined,following the administration of COX inhibitors and/or NC-1900 (1 ng/kg). When administered immediately, after the acquisition trial (Acq) in the PA task, indomethacin (20 mg/kg), a nonspecific COX inhibitor and NS-398 (10 and 20 mg/kg). a specific COX-2 inhibitor, but piroxicam (10 and 20 mg/kg), a specific COX-1 inhibitor, decreased the latency on the retention trial (Ret). The mnemonic effect of 1 mg/kg NC-1900 on the Ret at the PA task was also inhibited the administration of either indomethacin (20 mg/kg) or NS-398 (20 mg/kg) but not by piroxicam. However, when 20 mg/kg indomethacin and NS-398 were administered 3 h after the Acq, the increase in Ret latency induced by NC-1900 was not inhibited. These results suggested that the action of NC-1900 on the early stage of memory formation in the PA task may be modulated by endogenous COX-2 but not by COX-1. (c) 2007 Prous Science. All rights reserved.