To establish a technique for chronic renal vein catheterization in the primate, PE 240 catheters were introduced into the renal vein through the vena cava and led extraperitoneally to a subcutaneous pocket in the back in 17 baboons. After five to seven days, 14 were patent. Values for para-aminohippurate extraction EPAH and for oxygen tension (Po2) and pressure gradients between the renal vein and cava are presented. The mean EPAH was 81.4% ± 5.3 (95% confidence limits [CL]). The mean renocaval pressure gradient was 18.0 mm Hg ± 5.2 (95% CL). The mean renocaval Po2 gradient was 6.0 mm Hg ± 3.3 (95% CL).