Melatonin injected immediately after extinction training has no effect on extinction learning. (A) Schematic of the behavioral procedure used. Behavior procedure involved four training phases: habituation, fear conditioning, extinction training and extinction test. Rats were administrated melatonin immediately after extinction training. (B) Percent freezing to the CSs was shown for the melatonin group (n = 6) and the vehicle group (n = 6) during extinction training. (C) Percent freezing to the CSs during extinction testing. Mel = melatonin group, Veh = vehicle group. All data are represented as mean ± SEM.