The sulfhydryl reagents Hg++ and p-chloromercuribenzene sulfonate (PCMBS) at millimolar concentrations reduced the mucosal entry of sugars and amino acids to 80-90% of control levels within several minutes. Based on 50% levels of inhibition, Hg++ proved to be 20 and 10 times as potent as PCMBS in blocking sugar and amino acid transport, respectively; both systems were equally sensitive to Hg++. Concomitant measurements of 203Hg-PCMBS demonstrated a progressive tissue uptake, which, unlike inhibition, did not saturate with increasing times of exposure, thus suggesting appreciable epithelial entry with prolonged exposures (less than 30 min at 1 mM). At similar dose levels, no significant change in mucosal Na+ entry was detected. Inhibition was not reversed by 30-min washes in cholinesalt solutions; however, 10-min exposures to dithiothreitol [10 mM] reversed Hg++ and PCMBS inhibition by 40 and 100%, respectively. Alanine and galactose influx kinetics measured at concentrations of 0-100 mM exhibited a linear or diffusional entry component in addition to the usual saturable component for both control and Hg++-treated ileum. The presence of a diffusional term in the flux equation resulted in two sets of parameters giving nearly equal fits to these measurements. It was shown that this ambiguity could be resolved by determining the change in diffusional entry with Hg++ treatment. A 20-min exposure to 0.5 mM Hg++ caused an increase from 0.050 and 0.045 to 0.064 and 0.070 cm/hr in the coefficient of diffusional entry for alanine and galactose, respectively. On the basis of this increase, it is argued that Hg++ causes a decrease in Jmax and little change in Km for both transport mechanisms. This analysis has a general bearing on kinetic measurements of transport in which passive fluxes are comparable to those mediated by specific pathways. The alanine results are consistent with bimolecular reactions between mercurial and two membrane inhibitory sites, each producing approximately 40% reduction in membrane translocation rate. The estimated reaction rate constants were 5.0 and 0.4 mM min.