The exchange of tritium into water from saxitoxin molecules that were radiolabeled at the C-11 methylene position was measured at 37 degrees in solution and in suspensions of brain membranes. High concentrations of membrane receptors were used to assure that more than 80% of the total saxitoxin (STX) present was specifically bound. The amount of back-exchanged tritium was determined either by measuring the radioactivity remaining in the STX, using a second binding assay, or by measuring the tritium in water using ion-exchange chromatography. The results show that the back-exchange is accelerated in the presence of the membranes, and that this is attributable solely to the nonspecific toxin binding. Little change in the back-exchange rate over that in solution occurs in specifically bound toxin molecules. These results place certain restrictions on the possible bonds and configurations of receptor-toxin complexes.