Glutaraldehyde-carbohydrazide polymer (GACH) was used to embed olfactory tracts, trapezoid body, and sciatic nerves of 9-, 10-, and 49-day old mice 2 h, 24 h, and 6 wk (respectively) after the intraperitoneal administration of [1,2-3H]cholesterol. Greater than 94% of radioactive cholesterol was retained in the GACH-infiltrated brain 24 h or more after injection. The fine structural preservation of both central and peripheral nervous tissues was excellent. Quantitative analysis of electron microscope autoradiographs demonstrated that [1,2-3H]cholesterol is limited to blood vessel walls and lumen within the central nervous system at 2 h and 6 wk postinjection, but neurons and neuropil also contain the labeled cholesterol. The thickest myelin sheaths in the adult mice appear to be uniformly labeled throughout their width. No relationship of the retained [1,2-3H]cholesterol to the node of Ranvier was found in the adult sciatic nerve.