The distribution and pharmacological action of propiverine, a bladder spasmolytic agent, are affected by the extent of plasma-protein binding. Because attempts to assess the albumin-binding of propiverine have produced conflicting results, the binding parameters of the drug and some of its metabolites to serum albumin in man have been re-evaluated. In man propiverine is bound to serum albumin at a single site with high affinity (KA1 = 1.45 x 10(4) L mol-1) and at least two sites with low affinity (KA2 = 2.5 x 10(2) L mol-1). The metabolites of propiverine, namely M2 (dealkylated propiverine), M5 (the N-oxide of propiverine) and M6 (the N-oxide of M2), are less firmly bound to serum albumin; this is considered to be non-specific binding. Binding experiments with human serum revealed that there are additional binding proteins. At therapeutic plasma levels the extent of binding was calculated to be 90, 15, 60, and 20% for propiverine and the metabolites M2, M5, and M6, respectively. The strong binding of propiverine to serum proteins controls its availability to the liver. Because the metabolites are not tightly bound to serum proteins, after metabolism of propiverine its metabolites are easily eliminated.