1. Endoneurial blood flow (EBF) in the sciatic nerve of rats aged 2‐12 weeks was studied using microelectrode H2 polarography. 2. EBF is highest in 2‐week‐old rats and progressively declines during development. Mean arterial pressure (MAP) is low at 2 weeks of age, gradually increases through the next 4 weeks, and is relatively constant thereafter. The decrease in EBF, in spite of an increase in MAP, occurs because the endoneurial vascular resistance is increasing faster than the MAP. 3. The higher EBF in younger rats is not due to the smaller diameter of their nerves. Sural and tibial nerves of 12‐week‐old rats, with diameters comparable to that of the sciatic nerve of a 3‐week‐old rat, have EBFs similar to that of the sciatic nerve of a 12‐week‐old rat. 4. There was no compelling evidence of autoregulation of EBF in 3‐week‐old rats over a MAP range from ‐40 to +30 mmHg of the normal value. 5. The increase of nerve vascular resistance with maturation is probably due to a decrease in capillary density and, to a lesser extent, to an increase in plasma viscosity and haematocrit. 6. The higher EBF in immature rats is likely to be a developmentally adaptive mechanism which permits greater blood‐nerve exchange of material to accommodate the greater metabolic needs of rapidly elongating and myelinating axons and proliferating Schwann cells.
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