The muscarinic receptors responsible for two effects elicited by McN-A-343, i.e. the relaxation of the rat duodenum and the inhibition of the twitch contraction of rabbit vas deferens, were investigated by use of derivatives of 4-diphenyl acetoxy-N-methyl piperidine methobromide (4-DAMP). Both receptors had been previously identified as M1 on the basis of the high affinity shown towards pirenzepine. Schild analysis of antagonism revealed that the affinities of 4-DAMP and three of its analogues in the rat duodenum were significantly different from those estimated in rabbit vas deferens. These data indicate that distinct receptor subtypes mediate duodenal relaxation and vas deferens inhibition of twitch contraction and suggest that receptors classified as M1 by means of pirenzepine affinity constitute a heterogeneous population.