The critical period for sexual differentiation of the brain of the developing lamb occurs before birth. Exposure to steroids during this sensitive period in midgestation alters the control of GnRH secretion after birth. The present study examined neurons immunolabeled for GnRH in male and female lambs during the critical period for sexual differentiation to determine if these neurons are sexually dimorphic. Neuron number, anatomical distribution, and the number of neuronal processes of GnRH-containing neurons from midgestation (85 days) male and female fetuses were compared (n = 5 each). Immu-noreactive cells were labelled using LR-1 as the primary antiserum, followed by a biotinylated secondary antibody and the formation of an avidin-biotin-horseradish peroxidase (HRP) complex. The HRP was visualized histochemically using diaminobenzidine as the chromagen. GnRH neurons were localized in 60 µm coronal sections from the area of the diagonal band of Broca to the rostral mammillary bodies. The neurons were classified as unipolar, bipolar, or multipolar, according to the number of neuronal processes. The number of neuronal processes, the pattern of distribution, and the estimated total number of GnRH neurons in male and female fetuses was similar (p > 0.05). Furthermore, these parameters were equivalent to those reported for the adult female. These data indicate the GnRH neurosecretory system matures prior to midgestation in the sheep, and in a similar manner in males and females.