Bidirectional fluxes of 3H-glucose and 22Na+ in Hymenolepis diminuta were examined. Pulse-chase techniques involving 2-min incubations with 3H-glucose or 22Na+ followed by 15-sec post-incubations with unlabeled glucose and Na+, were used to estimate efflux rates of the labels after cell penetration. Effect of Na+ concentration (10 and 50 mM) on the efflux rates was determined. Efflux of 3H-glucose in the 15-sec postincubations was unaffected by Na+ concentration and appeared to occur by diffusion. 22Na+ efflux, on the other hand, was markedly greater in 50 mM Na+ than in 10 mM Na+ and appeared to occur by countertransport. This finding suggested that a coupling coefficient (22Na+ influx/3H-glucose influx) might be underestimated at a 22Na+ concentration of 50 mM because of the 22Na+ exchange phenomenon. The loss of 22Na+ due to exchange was minimized by decreasing the incubation time from 2 min to 15 sec; 22Na+ efflux in 15 sec relative to influxes of both 10 and 50 mM 22Na+ were negligible. Using 15-sec incubations we obtained coupling coefficients of 2.09, 2.24, and 1.89 in Na+ concentrations of 5, 25, and 50 mM, respectively, suggesting that the coupling process is not dependent on the Na+ concentration in the ambient medium.