Concentration–response curves for the inhibitory effects of L-arginine (0.01–4.7 mM), L-citrulline (0.01–4.7 mM) and SIN-1 (1–100 μM) on [3H]ACh release from motor nerve terminals stimulated with 5 Hz-trains (750 pulses). Abscissa, log of the concentration (M) of L-arginine (0.01–4.7 mM, squares), L-citrulline (0.01–4.7 mM, circles) and SIN-1 (1–100 μM, triangles), applied 15 min before S2. Ordinate, percentage change in S2/S1 ratio as compared with the S2/S1 ratio in control experiments. Zero per cent represents identity between the two ratios, negative values indicate inhibition of evoked [3H]ACh release. Each point represents the mean±s.e.m. of 4–11 experiments.