The mechanisms involved in the transport of the organic cation N1-methylnicotinamide (NMN) were investigated in the isolated perfused rabbit S2 proximal tubule. NMN underwent a small reabsorptive transport which appeared to be passive. NMN secretory transport was saturable, inhibited by mepiperphenidol (10(-4) M) and presented a relatively low apparent affinity (apparent Km of 852 microM) for the organic cation transport system, with transport against a concentration gradient being observed only at low flow rates. Acidification of the perfusate, by either buffering it at pH 6.8 with MES, or by bubbling the bath with a mixture of 80% O2-20% CO2, resulted in a decrease, rather than an increase, of NMN secretion. Carbonic anhydrase inhibition with ethoxyzolamide (10(-4) M in the bath) did not modify NMN secretion. Addition of 20 to 40 microM NMN in the perfusate also did not change NMN secretion. Proton or organic cation counterexchange seemed therefore not to play a major role in NMN apical step of secretion, the basolateral step appearing to be the general organic cation system of transport for which NMN shows a low affinity.