The percentage of neurons undergoing necrosis during foetal and early post-natal development in normal and Sprawling (Swl) mutant mice was determined. Two major peaks in the occurrence of necrosis were found at the 13th and the 19th foetal days in both groups of mice but at all stages the incidence was greater in Swl. Electron microscopically the first degenerative changes seemed to involve the perikaryon while many of the cells undergoing necrosis in Swl already showed the characteristic abnormalities previously described in that mutant.