To investigate how the substitution of NH2 for CH3 affects the activity of three, potent, semirigid nicotinic agonists, carbamyl analogues were synthesized. The carbamyl agonists were 1-methyl-4-carbamyl-1,2,3,6-tetrahydropyridine methiodide (1), 1-methyl-4-carbamylpiperidine methiodide (2), and 1-methyl-4-carbamylpiperazine methiodide (3). Their potencies (reciprocals of the equipotent molar ratios) at the frog neuromuscular junction with reference to carbamylcholine were 0.77, 0.052, and 0.15, respectively. The acetyl analogues were more potent by factors of 65, 175, and 17, respectively. Explanations for this variable reduction in activity were sought by using computer-assisted molecular mechanics and calculations of electrostatic potential contours. Bioactive conformations of 1-3 were assigned on the basis of a well-supported pharmacophore and the ground-state conformation of the highly potent (50 times that of carbamylcholine) prototype, isoarecolone methiodide (4). Agonist 3 and its acetyl analogue superimposed closely in their ground-state, bioactive conformations, and the differences in their electrostatic potential contours were the least among the three pairs. Accordingly, their potencies differed the least. Agonists 1 and 2 both showed greater differences (with respect to their acetyl analogues) in their electrostatic potential contours and greater differences in potency. Agonist 2, in addition, could achieve the bioactive conformation only at the expense of 2.8 kcal mol-1, and, correspondingly, its activity relative to its acetyl analogue was lowest of all.