creted, do not penetrate biological membranes . It is, however, known that specialized transport mechanisms for endogenous glucuronides exist in the kidney and the liver (17). In view of the present findings relative to erythrocyte sequestration, it is tempting to suggest that the same o r similar transport mechanisms may be involved in actively ti,ansporting the conjugated inetabolites across the erythrocyte and possibly even other biological membranes. Supporting this contention is the ubiquitous existence of pglucuronidase in tissues. the reason for which remains unexplained. Could i t be that the active transport system controlling the transfer of glucuronides across the cell membranes is coupled with the pglucurcnidase system'! It is conceivable that following the active transport of the conjugated molecules, for examplc a t the biophase, there is an instantaneous hydrolysis and thus liberation of the drug or metabolite molecules which may then exert their pharmacological effect. Studies now i n progress are aimed at investigating the validity of the belief that conjugation may have important biological significance in membrane transport of pharmacologically active molecular species. The preliminary data on human subjects indicate that the urinary elimination of the conjugated ehlorpromazine metabolites was significantly inhibited by penicillin which is known to he actively secreted by the tubular cells.