Experiments were carried out in urethane-anaesthetised rats to investigate whether GABA is involved in mediating inhibition of neuronal activity in the dorsal half of the periaqueductal grey matter (PAG) after stimulation of the serotonin-containing projection to the PAG from nucleus raphe obscurus (NRO). Multibarrelled micropipettes were used to make recordings from 42 neurones in the dorsal half of the PAG. Most (n = 36) cells were quiescent. Their firing rate was therefore raised to 10-16 Hz by continuous iontophoretic application of DL-homocysteic acid (DLH) in order to facilitate the study of inhibitory events. Iontophoretic application of GABA (0-10 nA) silenced every neurone tested (n = 42), and the effect was blocked by the GABAA antagonist bicuculline (BIC, 10-20 nA; 15/15 cells). BIC also produced an increase in ongoing activity in 14 of 15 cells, indicating the presence of inhibitory GABAergic tone. Iontophoretically applied serotonin (5-HT; 10-70 nA) also inhibited ongoing activity in 9 of 11 cells. The effect of 5-HT was not blocked by BIC. In six of seven cells, microinjection of 100-200 nl DLH into NRO produced a 72.3 +/- 9.4% decrease in neuronal firing rate which was maximal 112 +/- 18 s after the start of the injection and lasted for a total of 313 +/- 63 s. In five of six cells, the raphe-evoked inhibition was blocked by BIC. It is suggested that activation of the serotonergic projection to the PAG from NRO engages GABA-containing interneurones within the PAG which mediate the inhibitory effects of raphe stimulation.