The efflux of [3H]purines from cultured sympathetic neurons prelabelled with [3H]adenine is accelerated 2-3-fold within hours of nerve growth factor (NGF) withdrawal and is reduced by readdition of NGF. Addition of 8-(4-chlorphenyl-thio) cAMP, which delays neurite degeneration, reduced the enhanced efflux of purines, as did the addition of cycloheximide, MgCl2 and the protease inhibitor tosyl-L-lysine chloromethyl ketone. Colchicine accelerated purine efflux and neurite degeneration but 2-deoxyglucose increased purine efflux without inducing degeneration, suggesting that ATP reduction itself is not the cause of neurite degeneration. The increase in purine efflux is thus an early biochemical event that has diagnostic value for the study of NGF action since deprivation is detected well before irreversible changes become established.