The purpose of the present study was to investigate the role of pineal gland (melatonin) on parturition time, luteal function, and fetal growth in pregnant rats. Cycling rats were subjected to pinealectomy or sham operation under ether anesthesia; and pinealectomized rats immediately underwent implantation of a melatonin capsule (PINX + Mel group) or a vehicle-containing capsule (PINX group), and sham operated rats also underwent implantation of a vehicle-containing capsule (control group). All rats were maintained under the same photoperiod conditions (14 L:10 D) and were induced pregnancy. Blood samples were obtained on days 7, 12, 15, 17, 19, and 21 of pregnancy to measure serum progesterone concentrations, and parturition times were recorded on days 22 and 23. In the next experiment, pregnant PINX rats received subcutaneous injection of melatonin (10 microg/body) at 08:00 h (PINX + 8 h group) or at 20:00 h (PINX + 20 h group) from day 15 to the end of pregnancy, and parturition times were recorded. Parturition times of rats in the PINX group, the PINX + Mel group or the PINX + 8 h group, but not the PINX + 20 h group, were significantly different compared with those in the control group. Pinealectomy or melatonin implantation did not affect serum progesterone concentrations during pregnancy or the number and weight of fetuses or corpora lutea. The present results indicate that pineal gland (melatonin rhythm) synchronizing with photoperiodic rhythm is likely to be an important determinant of parturition time, but it does not affect progesterone production or fetal growth in pregnant rats.