Acetylcholinesterase (AChE) is secreted from various brain regions such as the substantia nigra, where levels of this molecule are disproportionately higher than those of choline acetyltransferase. It is thus possible that AChE may have alternative, noncholinergic functions, one of which could be in development. Indeed, several recent studies have already demonstrated a neurotrophic action of AChE independent of hydrolysis of acetylcholine. In the developing nervous system the dominant forms of AChE differ from the tetramers (G 4 ) that prevail in maturity, in that they are lower molecular weight monomers (G 1 ) and dimers (G 2 ). Therefore, the aims of this study were to explore the neurotrophic role of AChE by comparing the effects of mouse recombinant G 1 and G 4 AChE on the survival and development of midbrain tyrosine hydroxylase immunoreactive neurons. Butyrylcholinesterase (BuChE), which also hydrolyses acetylcholine, and basic fibroblast growth factor (bFGF), an established trophic factor for midbrain neurons, were also tested. bFGF had no significant stimulatory effect: moreover, BuChE was also inefficacious, suggesting that the action of AChE was independent of its catalytic site. In contrast, mouse recombinant G 1 and G 4 AChE both increased the survival as well as the outgrowth of the cultured neurons. However, G 1 AChE was more potent than G 4 AChE suggesting that developmental forms of AChE exist. The implications of this finding for physiological and pathological functioning of the nervous system are discussed.