Glutaraldehyde fixation and osmication of rat sciatic nerve and human sural nerve did not appreciably alter the apparent weight of collagen in nerve fasciculi as determined by hydroxyproline analysis of hydrolysed samples. The collagen content of autopsied peripheral nerve is unaltered at least up to 24 h after death. The mean endoneurial weight of collagen per fascicle in rat sciatic nerve was 123 mug (all results expressed for 1 cm length). In human sural nerve the mean weight was 57 mug and the mean fascicular volume 1.75 mm3. Human endoneurial collagen fibrils gave a unimodal distribution of diameters with a peak at 40 nm.