The aim of this study was to evaluate the net changes in bone mineral density (BMD) during the reproductive cycle, and their relation with changes in serum calcium (Ca), phosphorus (P), PTH and calcitonin levels in rats. Twenty-seven female Wistar rats were included in this study. They were divided into three groups as pregnant, lactating and control groups. BMDs of lumbar vertebrates, femoral and tibial bones, and Ca, P, calcitonin and PTH levels were measured at the end of pregnancy, at the end of lactation and in nulliparous controls. In the pregnant group, the BMDs of rats were significantly higher in lumbar vertebrates, femoral and tibia bones than those of the control group (p < 0.05). Their PTH and Ca levels were significantly lower than the control group (p < 0.05). However, no statistically significant difference was found regarding P and calcitonin levels when compared to those of the control group. In the lactating group, the BMDs were significantly lower in lumbar vertebrates, femoral and tibia bones than those seen in the control and pregnant groups (p < 0.05). Ca and PTH levels were significantly higher in lactating rats than in those of pregnant rats (p < 0.005). Normal pregnancy increases BMD in rats, whereas lactation decreases it. Change in PTH levels is supposed to contribute to the mineralization and demineralization of the skeleton during pregnancy and lactation, respectively. (c) 2005, Editrice Kurtis.