1. The action of iontophoretic applications of 5HT on granule cells of the dentate gyrus was studied in the rat hippocampal slice with intracellular electrodes. 2. The inhibition produced by 5HT was accompanied by a depolarization of the membrane and a substantial decrease in input resistance. 3. 5HT was only inhibitory when applied close to the soma and had no effect when applied to the dendritic region of the cell. 4. The reversal potential for the 5HT response was estimated to be 15.5 +/- 6 mV depolarizing with respect to resting membrane potential; increasing intracellular Cl- shifted the reversal potential to a more positive value. 5. Equipotent applications of GABA applied from adjacent barrels of the same multibarrel electrode evoked similar responses with an identical equilibrium potential.