THE aim of this study was to assess the effect of potential target tissue on regenerating neurones of the snail Lymnaea stagnalis using the three-dimensional collagen gel culture system. Mammalian type I collagen supported the regenerative outgrowth of snail neurones, and the neurofilament antibody SMI31 specifically labelled regenerating processes both within the gel and those growing over the surface of the ganglia. Using these techniques we tested the effect of co-culturing ganglia with either additional nervous tissue, previously shown to produce trophic substances, or buccal muscle on both the amount and direction of outgrowth. We conclude that, under the conditions used, neither target tissue provided trophic or tropic support in collagen gel cultures.