Three lines of evidence indicate that intravascularly injected horseradish peroxidase labels dying neurones selectively in the developing isthmo-optic nucleus: (i) in unoperated embryos, labelled neurones are found only during the period of normal neuronal death; (ii) some of the labelled neurones are manifestly necrotic; (iii) an intraocular injection of colchicine causes essentially all the isthmo-optic neurones to degenerate, and then they almost all take up peroxidase. However, deafferenting the developing isthmo-optic nucleus by a tectal lesion increases the loss of neurones whilst scarcely affecting the number labelled; this implies that the neurones which die because of the deafferentation either do not take up the peroxidase or are phagocytozed exceptionally rapidly.