We have studied the evolution of acetylcholinesterase molecular forms during the embryonic development of Torpedo marmorata, in the electric organs and in the electric lobes of the central nervous system. In the early stages of development (35 mm embryos, 'myogenic phase' of electric organ development), globular forms of acetylcholinesterase (G(4) and G(2)) are abundant in both tissues and the collagen-tailed form A(12) is already present. In the electric organs, this form accumulates rapidly after the 55-60 mm stage ('electrogenic phase'), when synapse formation first commences. Although the molecular characteristics of the collagen-tailed forms, and particularly their aggregation properties, do not appear to change during development, their solubilization requires higher concentrations of MgCl(2), as the electrocytes mature, suggesting that they become more tightly integrated in a better organized basal lamina. The smaller collagen-tailed form A(8) shows a transient increase which coincides approximately with the maximal accumulation of A(12), suggesting that it is an intermediate in its synthesis. The accumulation of the hydrophobic G(2), which eventually becomes predominent in the adult electric organs, lags behind that of A(12). The functional significance of this important fraction of acetylcholinesterase is therefore not that of a pool of precursor for the synthesis of A(12). In the electric lobes, the tetrameric form (G(4)) is abundant during development, as well as G(2) and G(1) at certain stages, but the A(12) form is predominant in the adult.