The gamma-aminobutyric acid(A) (GABAA) receptor is a ligand-gated ionophore involved in synaptic inhibition. Biochemical and molecular biological studies indicate that considerable receptor heterogeneity exists, but physiological differences between inhibitory GABAA synaptic responses have not been identified in the brain. The present report describes two anatomically segregated GABAA-mediated synaptic currents in the hippocampal CA1 region that have distinct physiological, pharmacological, and functional properties. GABAA,fast enters at or near the cell body, decays rapidly (3-8 ms), is blocked by furosemide, and rapidly curtails the excitatory response. GABAA,slow enters far from the cell body, decays slowly (30-70 ms), is not blocked by furosemide, and underlies the conventionally recognized early inhibitory postsynaptic potential. The receptors producing these responses may represent subtypes of the GABAA receptor.