1. Electrical and pharmacologic properties of glycine-induced currents were investigated in single hypothalamic neurons acutely isolated from young and adult rats by the use of a "concentration-clamp" technique, which allows both internal perfusion and rapid application of an external solution under single-electrode voltage-clamp. 2. The glycine-induced current reversed at the Cl- equilibrium potential (ECl), and a 10-fold decrease of extracellular Cl- with a large impermeable anion resulted in a 53 mV shift of the glycine reversal potential (EGly). 3. Glycine-induced Cl- currents (ICl) increased sigmoidally in a concentration-dependent manner with a Kd of 9 X 10(-5) M at a Hill coefficient of 1.8. Current inactivation occurred completely at all concentrations within 10 s. EGly remained unchanged during continuous application of glycine, suggesting that the inactivation process is because of desensitization. 4. The glycine-induced conductance exhibited a striking voltage dependency at membrane potentials more negative than -50 mV and reached a steady state value when hyperpolarized beyond -110 mV. 5. Both the activation and inactivation phases of glycine-induced ICl are described by double exponential (fast and slow components) functions with the concentrations used. All four time constants decreased with increasing glycine concentration. 6. The slow time constant of the current decay induced by glycine increased with depolarization and decreased with hyperpolarization, indicating that the rate of desensitization is considerably voltage dependent. The fast decay showed little voltage dependency. 7. Recovery of the glycine response after complete desensitization consisted of two components. 8. The blockade of the glycine response by strychnine and picrotoxin was noncompetitive.