This study was conducted to determine the effect of gestational diabetes on the neuronal density of CA1 and CA3 subfields of the hippocampus in Wistar rat offspring. On day 1 of gestation, 10 dams were randomly allocated into two control and diabetic groups. Five animals in the diabetic group received 40 mg/kg/b.w. of streptozotocin (intraperitoneally) and the control animals were received normal saline. Six offspring of each of the gestational diabetics and controls were randomly selected in postnatal days 7 and 21. The infants were scarified and coronal sections were taken from the right dorsal hippocampus and stained with cresyl violet. The number of pyramidal cells per 10000 μm(2) area and the thickness of layers of hippocampus in CA1 and CA3 were evaluated. In postnatal day 7, the number of pyramidal neurons in CA1 significantly reduced from 118.82 ± 8.0 in the control group to 84.71 ± 3.3 neurons in gestational diabetic group, and in postnatal day 21 it significantly reduced from 112.71 ± 6.9 in the control group to 91.52 ± 8.5 in the gestational diabetic group. Also, the number of pyramidal cells of CA3 on postnatal day 7 significantly reduced from 90.33 ± 8.1 in the control group to 62.86 ± 7.2 in the gestational diabetic group, and in P21 the number of pyramidal cells significantly reduced from 78.33 ± 2.4 in the control group to 61.7 ± 9.5 cells in the diabetic group. In CA1 and CA3 the thickness of the pyramidal layer on postnatal days 7 and 21 non-significantly increased in gestational diabetics in comparison with the controls. This study showed that uncontrolled gestational diabetes reduces the pyramidal neurons of the hippocampus in rat offspring.