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 μm² 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.