The aim of this study was to determine physiological value of platelet serotonin (5-HT) and its variations in the group of women in term pregnancy and after birth. Obtained results were compared to the platelet 5-HT level in nonpregnant women group. Determination of normal level of 5-HT in pregnancy and after could help in its further measurement and evaluation of different psychologic and psychiatric disorders related to pregnant and postpartal period, including better understanding of mood changes after the birth. A total of 137 healthy Croatian women were enrolled in the study--82 of them were pregnant and 55 were not. Their blood was sampled and the platelet serotonin concentration was determined. In pregnant women the blood was sampled twice: at term pregnancy, and soon after birth. The mean value of 5-HT in pregnant women was 1.209 nmol/mg protein, after the delivery 1.045 nmol/mg protein, and in non pregnant 1.088 nmol/ mg protein. The concentrations were significantly different in those three groups. We did not find differences in 5-HT levels in groups divided by age.