Pyknotic nuclei in the external granular layer (EGL) of the rat cerebellum accounted for 0.6-0.8% of the total cell population in the first and third post-natal weeks and 1.4% at day 12. A wave of cell death induced by hydroxyurea showed an exponential decrease after 12 hr, from which a mean duration for pyknoses of about 6.5 hr was calculated. Undernutrition delayed the clearance of pyknotic nuclei following hydroxyurea, the mean duration being 24.5 hr. It is concluded that cell death does not significantly influence cell acquisition in the developing cerebellar EGL, and that the increased pyknotic index seen in the EGL of undernourished rats is a consequence of delayed clearance rather than an increase in the number of cell deaths.