The involvement of membrane sulfhydryl groups in the uptake of adenosine and thymidine was examined in rat thymocytes pretreated with 6,6'-dithiodinicotinic acid (CPDS) and N-ethylmaleimide (NEM). CPDS, which is known to react uniquely with external membrane sulfhydryls, under short incubation conditions, did not significantly affect the uptake of adenosine and thymidine. Formation of cAMP in nonstimulated and adenosine-stimulated cells was also unaltered by CPDS. However, inhibition of adenosine uptake by competitive inhibitor, dipyridamole, was significantly stronger when the cells were pretreated with CPDS. Preincubation of cells with NEM showed differential sensitivity of adenosine and thymidine uptake, depending on concentration of this sulfhydryl alkylating agent. The results suggest the involvement of NEM-accessible sulfhydryls in membrane transport of adenosine and thymidine. Dual effect of NEM on nucleoside transport may be related to the complexity of nucleoside carrier(s) or to the existence of different nucleoside carriers within thymocyte membranes. On the other hand, the easily accessible, external membrane -SH groups which can be blocked with CPDS, are not essential in thymocyte nucleoside transport but they appear to be situated at a site which interacts with the membrane transport system of nucleoside.