To investigate the influence of blood extraction conditions on the renin-angiotensin system in rats, plasma renin activity (PRA) and plasma renin concentration (PRC) were measured in blood samples obtained by different methods. PRA and PRC in samples obtained by chronic catheterization, cardiac puncture without anesthesia, and decapitation immediately following light ether anesthesia were not significantly different from those obtained by simple decapitation (control group). In contrast, PRA and PRC in samples obtained by cardiac puncture and cavernous sinus puncture after light ether anesthesia were significantly (p less than 0.01) higher than those obtained in the control group. There was a significant direct correlationship between PRA and PRC in all samples studied (r = 0.87, p less than 0.001). The present results suggest that light ether anesthesia increases renin levels, except when blood samples are taken by decapitation, and that chronic catheterization and cardiac puncture are the choice blood extraction methods to evaluate the renin-angiotensin system in rats.