In vivo studies of rat sciatic nerves in models of immediate and delayed repairs demonstrated the viscoelastic properties of the nerve and the inverse correlation between nerve blood flow and tension. In both the proximal and distal segments of the divided nerve in models of immediate and delayed repairs, the nerve blood flow decreased approximately 50% with substantial recovery in 30 minutes after 8% elongation, whereas 15% elongation produced approximately an 80% reduction in blood flow with minimal recovery. However, the baseline blood flow of the nerves in the delayed-repair model was nearly two times higher than that of the acutely injured nerves. Maximal decrease in nerve tension and corresponding increase in blood flow occurred within the first 20 minutes after elongation. The suture pull-out with failure of the repairs occurred at more than 15% elongation for all nerves. The previously divided nerves had a sixfold greater decrease in length than the acutely divided nerves (p < 0.02). For repairs of large nerves where vascular ingrowth is likely to be incomplete, elongation of more than 8% may cause ischemia that is detrimental to nerve regeneration. Mechanical failure of the repairs occurs after elongation of 16% to 17%. The combination of nerve ischemia and mechanical failure of suture repairs suggests that surgeons should be careful to limit the use of elongation in acute and delayed repairs.