The substantia nigra pars reticulata (SNR) plays important roles in movement and, in an age- and sex-dependent manner, in seizure control. GABAergic synaptic transmission is critical in both normal development and seizures. In many neuronal types it is excitatory early in development and later switches to the mature hyperpolarizing type. We assessed the time course of the switch of GABAA receptor-mediated postsynaptic currents (PSCs) in anterior SNR neurons of male and female developing rats using the gramicidin perforated patch clamp technique. The switch occurred in males around postnatal day (PN) 17 and in females around PN10. This sex dimorphism may play a role in several other recognized sex differences in the development of SNR and in its regulatory role in seizures.