The effect of substituted pyridines on the response of single pyridine-sensitive cells from crayfish walking legs was investigated electrophysiologically. Sevenp-substituted pyridines, acting reversibly, were identified as specific antagonists at the pyridine receptor. The maximum saturation frequency of the response to agonists was reached in the presence of antagonists but the dose–response curves of the agonists were shifted in parallel to the right along the concentration axis. Schild plots of three highly effective antagonists with six agonists were linear with a slope close to one, indicating competitive antagonism. The inhibition constants yielded a K1value of˜4–8μmol for the most effective antagonist, 4-(4-nitrobenzyl)pyridine, which had only one order of magnitude lower affinity than the most effective agonist 2,3′-bipyridyl. The antagonists 2,4′-bipyridyl and 4-benzylpyridine had K1values of 6–10μmol, followed by 4-acetylpyridine with a K1value of 30–70μmol. The rank order of inhibitory potency of the different antagonists was found to be the same for all units tested. Comparing electronic effects (Hammettαvalues and pKa values) of the substitutents inp- andm-position showed that inhibitory effectiveness decreased with a decrease in pKa and an increase inαPartition coefficients were determined for 10 agonists and the antagonists which were generally more lipophilic than the agonists. A hypothetical receptor