The jet-lag syndrome and the shift-work malaise take place by rapid changes in photoperiod and disturb people's life frequently. In the present study; we wanted to simulate these conditions. Control, pinealectomized and melatonin including silastic tube implanted Mongolian gerbils were respectively exposed into long (14L:10D), short (8L:16D), constant darkness (0L:24D) and constant light (24L:0D) photoperiods in order to investigate the effects of photoperiod, pineal gland and melatonin hormone in the photoperiodic reentrainment of this species. Gerbils showed an light/dark cycle entrained locomotor activity rhythm in long and short photoperiods, free-running in constant darkness and arrhythmic locomotor activity in constant light. The period lengths were similar to each other in the groups (p>0,05). Gerbils were reentrained by the changes in photoperiod nevertheless pinealectomy and melatonin implants did not make any significant influence on this reentrainment (p>0,05). Locomotor activity amounts varied in a group-photoperiod-dependent manner. These data suggest that photoperiod but not the pineal gland and constant release melatonin hormone is effective on the activity rhythm of the Mongolian gerbil.