An analysis of cellular kinetics in the NMRI-mouse after 12, 13, 15, and 17 days of gestation was obtained by means of tritiated thymidine autoradiography. After 12 and 13 days there is no significant difference in generation time between the lateral and the medial districts of the telencephalic roof. From 15 days of gestation onwards, the generation time in the lateral parts is significantly greater than in the medial wall regions. Simultaneously, on day 17 the growth fraction is drastically decreased in the lateral parts, while it remains close to 1 in the medial parts. Lengthening of the generation time during differentiation of the lateral wall is mainly due to an extension of the G1-phase and to a lesser degree also of the S-phase. Another significant contribution comes from the increasing length of the mitotic phase.