A quantitative study was made of the differential effects of the antifolate amethopterin (methotrexate) on the development of cerebral hemispheres, optic lobes and cerebellum in chick embryos. Most surviving embryos show a significant under-development at day 18 of incubation. Different brain parts respond differently (disproportionately) to the treatment: cerebellum--most drastically, optic lobes--least drastically (net weights). Within the cerebellum, Purkinje and other large cells respond less drastically than small cells. The embryo thus had highly significantly changed ratios of weights of various brain parts and of numbers of various cerebellar cells. Several mechanisms to explain these differential effects have been discussed.