A new class of substituted 1-phenyl-3-piperazinyl-2-propanones with antimuscarinic activity is reported. As part of a structure-activity relationship study of this class, various structural modifications, particularly ones involving substitution of position 1 and the terminal piperazine nitrogen, were investigated. The objective of this study was to derive new antimuscarinic agents with potential utility in treating urinary incontinence associated with bladder muscle instability. These compounds were examined for M1, M2, and M3 muscarinic receptor selectivity in isolated tissue assays and for in vivo effects on urinary bladder contraction, mydriasis, and salivation in guinea pigs. Potency and selectivity in these assays were influenced most notably by the nature of the substituent group on the terminal nitrogen of the piperazine moiety. Benzyl substitution was particularly advantageous in producing compounds with functional M3 receptor (smooth muscle) and bladder selectivity; it provided several candidates for clinical study. In vivo, 3-(4-benzyl-piperazinyl)-1-cyclobutyl-1-hydroxy-1-phenyl-2-propanone (24) demonstrated 11- and 37-fold separations in its effect on bladder function versus mydriatic and salivation responses, respectively. The corresponding 2-chlorobenzyl derivative 25 was more than 178-fold selective for M3 versus M1 and M2 muscarinic receptors. 3-(4-Benzylpiperazinyl)-1,1-diphenyl-1-hydroxy-2-propanone (51) was 18-fold selective for M3 versus M1 and 242-fold selective for M3 versus M2 receptors. It was also selective in guinea pigs, where it displayed 20- and 41-fold separations between bladder function and effect on mydriasis and salivation, respectively. In general, the results of this study are consistent with the proposition that the described piperazinylpropanones interact with muscarcinic receptors in a hydrogen-bonded form that presents a conformation similar to that apparently adopted by classical antimuscarinic agents.