1. The effect of cannabinoid receptor agonists was studied in guinea-pig myenteric neurones in vitro by use of conventional intracellular recording techniques. 2. Exposure of myenteric neurones of the S-cell type to the cannabinoid receptor agonists WIN 55,212-2 (100 nM) and CP 55,940 (100 nM) reversibly and significantly depressed the amplitude of fast excitatory synaptic potentials (fast e.p.s.ps) by 46% and 37%, respectively. 3. The depressant effect of WIN 55,212-2 and CP 55,940 on fast e.p.s.p. amplitude (expressed as the area above the amplitude-time curve (mVs)) was significantly greater than that of the vehicle, Tween 80, which had no detectable effect. 4. The inhibitory effect of WIN 55,212-2 appeared to be concentration-dependent over the range 1-100 nM. WIN 55,212-3, its (-)-enantiomer (100 nM), was inactive. 5. The cannabinoid CB1 receptor antagonist, SR141716A (1 microM), reversed the inhibitory effects of WIN 55,212-2 on fast e.p.s.ps in 38% of neurones tested (3/8) and acetylcholine (ACh)-induced depolarizations in 42% of neurones tested (5/12). 6. When tested on its own, SR141716A (1 microM) caused a 40-50% reduction in the amplitude of fast e.p.s.ps (n = 9). 7. WIN 55,212-2 reversibly depressed the amplitude of the slow e.p.s.p. and, in 2 out of 7 neurones, this effect was reversed by SR141716A (1 microM). 8. It is concluded that cannabinoid-induced inhibition of fast cholinergic synaptic transmission occurred by reversible activation of both presynaptic and postsynaptic CB1 receptors and that slow excitatory synaptic transmission can also be reversibly depressed by cannabinoids. Furthermore, it would seem that subpopulations of myenteric S-neurones and their synapsing cholinergic and non-cholinergic, non-adrenergic terminals are not endowed with cannabinoid receptors.