Auditory evoked middle latency responses recorded in the hippocampus (HAER), were monitored in alert, gently restrained rats with chronic indwelling electrodes and cannulae. Intrahippocampal (i.h.) injection of 5-hydroxytryptamine (5-HT, 10 micrograms) reduced the amplitude and increased the latency of the N28 and P55 peaks of the HAER. An early (P18) negative peak was unaffected. Buspirone (1 microgram, i.h. and 3 mg/kg, i.p.) had similar effects to those produced by i.h. 5-HT. RU 24969 (1 mg/kg, s.c.) also reduced the amplitude of the N28 peak of the HAER. Long-term treatment with buspirone for 14 days at a dose (0.5 mg/kg, i.p.) which when applied acutely did not produce any observable effect, caused an increase in the latency of both the N28 and P55 peaks. Direct i.h. injection of 5-HT into these chronically treated animals did not have any additional depressant effect on the HAER peaks. It is concluded that these serotoninergic agonists can modulate the later peaks of the HAER possibly via 5-HT1A receptors. In the case of buspirone there was evidence of an enhanced depressant effect following chronic treatment [corrected].