GABA is excitatory in the developing brain due to a positive shift of the chloride equilibrium potential relative to that found in adult neurons. Activation of GABAA receptors depolarizes neurons sufficiently to open L-type voltage-gated Ca2+ channels (J Neurosci 15:5065). During the time when GABA is excitatory, the brain is sexually differentiated under the control of gonadal steroids, primarily estradiol (E2). We reported previously that 10-10M E2 treatment from DIV 1-4, equivalent to embryonic day (ED) 15-ED 18, increased peak levels of free Ca2+ induced by 10μM muscimol in cultured cells imaged on ED 19 (SFN 1999 abstract #197.8). Using a similar hypothalamic culture system generated from ED 15 mixed-sex embryos, we have extended these data by imaging neurons at 8DIV, the equivalent of ED 22 (day of birth; DOB). Cells were loaded with the calcium indicator, Fura-2/AM, and free intracellular Ca2+ was measured. Peak Ca2+ levels were increased (61%) on ED 22 by treating neurons with 10-10M E2 from ED 18-...