Objectives: (1) Inosine uptake is mediated by a membrane localized inosine phosphorylase whereby ribose-1-p accumulates inside the vesicle while hypoxanthine is released outside the vesicle. The reaction is reversible and product inhibition patterns indicate that the sites for Hx and Inosine are on the outside of the vesicles while the site for Ribose-1-P is on the inner surface. (2) Hypoxanthine uptake is stimulated 10-fold by exogeneous addition in PRPP, accumulates as IMP and is lacking in vesicles from a HGPRT'ase, 6-thioguanine resistant mutant. In the parent cell line HGPRT is partially membrane associated. (3) Uridine uptake is diminished at confluence (uridine accumulates in the vesicles unaltered) in a contact inhibited cell line but not in its SV-40 transformed conjugate line. Though this experiment was performed in whole cells by Cunningham and Pardee who proposed growth control may be exerted at the membrane transport level several years ago, our results with isolated vesicles indeed demonstrate this to be the case for the first time. Furthermore the ability to observe biologic regulatory phenomena in the isolated vesicle reaffirms the usefulness of a simpler experimental system in which the key function remains intact than the whole cell. Our work has mostly been with L cells (including L929 se- which we grow on completely defined media--no serum) and with Balb/c 3T3 and SV-40 Balb/c 3T3. We have studied transport in CHO and BHK/BHK Py vesicles to a lesser extent.