The effects of P-4 and its active metabolites in human blood and urine and the effects of 1-methyl-4-piperidyl diphenylpropoxyacetate N-oxide [P-4(N----O)], 1-methyl-4-piperidyl benzilate N-oxide [DPr-P-4(N----O)] and 1-methyl-4-piperidyl benzilate (DPr-P-4) on isolated guinea pig urinary bladder were investigated. At doses of 10(-6) or 10(-5) M, P-4 shifted the dose-response curve for acetylcholine (ACh) to the right, and at a dose of 10(-5) M, it also inhibited the maximum response of ACh. At doses of 10(-5) M or more, P-4(N----O) inhibited the maximum response of ACh. DPr-P-4(N----O) or DPr-P-4 shifted the dose-response curve for ACh to the right at doses of 10(-6)-10(-4) M or at doses of 10(-7)-10(-5) M. P-4 at doses of 10(-6) M or more inhibited the KCl (100 mM)-induced contraction in a dose-dependent manner, and its potency was quite equal to that of terodiline. The inhibitory effect of P-4(N----O), DPr-P-4(N----O) and DPr-P-4 in the KCl (100 mM)-induced contraction were weaker than that of P-4. P-4 and P-4(N----O) had a dose-dependent inhibitory effect on K+-induced 45Ca influx in isolated guinea-pig urinary bladder.(ABSTRACT TRUNCATED AT 250 WORDS)