1. The inhibitory response of exogenously applied glycine was investigated in freshly dissociated rat nucleus tractus solitarii neurons under whole cell configuration using new perforated patch-clamp technique termed 'gramicidin perforated patch technique,' which maintains intact intracellular Cl- concentrations. 2. Using the gramicidin perforated patch technique, at a holding potential (V(H)) of -45 mV, glycine induced outward currents in a concentration-dependent manner with a EC50 of 4.0 x 10-5 M and at a Hill coefficient of 1.5. In contrast, using the nystatin perforated patch technique, glycine induced inward currents at the same V(H) in a concentration-dependent manner with an EC50 of 4.9 x 10-5 M and at a Hill coefficient of 1.2. 3. The glycine-induced outward currents were blocked by strychinine in a concentration dependent manner with an IC50 of 2.2 x 10- 8 M. The blockade was competitive. 4. The current-voltage relationship for the 10-5 M glycine response showed a clear outward rectification. 5. Ten- fold change of extracellular Cl- with a large impermeable anion resulted in a 65 mV shift of the reversal potential of glycine-induced currents (E(Gly)), indicating that the membrane behaves like a Cl- electrode in the presence of glycine. 6. The intracellular Cl- activity calculated from the E(Gly) ranged from 7.3 to 18.2 mM, with a mean value of 13.3 mM. 7. The values of E(Gly) in the individual neurons were significantly negative to the resting membrane potentials, suggesting the existence of active transport of Cl-.