Fetal alcohol exposed (FAE) animals exhibit physiological and behavioral deficits associated with reproduction including alterations in LH secretion and decreased sex behavior. Such deficits led us to examine the morphological characteristics and number of GnRH neurons in female rats prenatally exposed to ethanol. Sprague-Dawley dams (Harlan, Indianapolis, IN) were administered a fortified liquid diet (Sustacal) containing 35% ethanol derived calories from day 7 through parturition. Controls were pair-fed a similar isocaloric diet containing no ethanol during this period or allowed access to dry food pellets ad lib. Compared to both control groups, puberty was found to be significantly delayed in females that were exposed prenatally to ethanol. The median age of vaginal opening for chow-fed and pair-fed controls was 34-35 days compared to 38-39 days for FAE animals. A subgroup of these females was selected at 44 days of age on the basis of delayed onset of puberty and compared with pair-fed controls for the number and morphology of GnRH-immunoreactive staining (ir) neurons in the medial preoptic area (MPOA) and the diagonal band of Broca (DBB), regions which contain the majority of GnRH-IR cells in the rat brain. In both areas, light microscopic study revealed no differences in GnRH-IR cell number, nor were differences detected among the groups in the number of bipolar versus unipolar cells. However, a significant increase was observed in FAE brains compared to controls in the ratio of GnRH-IR neurons with irregular processes compared to smooth cell contours.(ABSTRACT TRUNCATED AT 250 WORDS)