Fertilized chicken eggs 24 to 72 hours old were incubated in cyclopropane 20–50 vol per cent for three to 12 hours. Zero to 48 hours after treatment the embryos were sacrificed and mitotic figures in the neural tube counted. Cyclopropane had two effects on cells in mitosis, increasing the ratio of cells in metaphase to cells in prophase, and decreasing the number of cells in mitosis. The former was seen primarily after exposure to low cyclopropane concentrations, the latter after high concentrations. Twenty-four hours after diseontinuation of treatment the distribution of cells in mitosis appeared normal.