The time course of the effect of pentylenetetrazol (PTZ)-induced kindling on acetylcholine release in the hippocampus of freely moving rats was investigated with the transversal microdialysis technique. The basal extracellular concentration of acetylcholine in the hippocampus was reduced significantly (-29%, P < 0.05) after 3 weeks, and the effect was maximal (-52%, P < 0.01) after 4 weeks and remained essentially unchanged during the remaining 4 weeks of PTZ treatment (30 mg/kg, i.p., 3 times/week), relative to vehicle-treated rats. The basal release of acetylcholine in the prefrontal cortex and in the striatum of kindled rats was unchanged compared with that of vehicle-treated rats. The specific binding of [3H]quinuclidinyl benzilate, a non-selective ligand of muscarinic receptors, was significantly increased (+29%, P < 0.01) in hippocampal membrane, but not in membranes prepared from the prefrontal cortex or striatum, of PTZ-kindled rats. Thirty days after discontinuation of PTZ treatment, both hippocampal acetylcholine output and the density of muscarinic receptors had returned to values characteristic of vehicle-treated rats, whereas seizure susceptibility did not differ significantly from that apparent 4 days after PTZ administration. These results suggest that the selective and transient decrease in acetylcholine output and the parallel increase in the density of postsynaptic muscarinic receptors in the hippocampus may play a role in facilitating the development of kindling rather than in the maintenance of the kindled state.