The distribution of cells of different labelling intensities in the anterior forebrain of adult mice injected with tritiated thymidine at daily intervals during prenatal life was determined by mapping the location of labelled cells on enlarged photographs of autoradiographed sections. The isocortical arc was subdivided into an arbitrary number of radially orientated units. Each radial unit was found to have a similar sequence of arrival and distribution of labelled cells; the ventrolateral units, however, entered and completed the sequence ahead of dorsomedial units indicating the presence of a wave of differentiation spreading in this direction across the generative layers giving rise to cortical neurons. An attempt was made to identify (from differential grain counts) comparable samples of first and second generation cells produced after each pulse of labelled thymidine. The changing ratio between the two generations suggested that there may be two peaks in neuron birth during the generative period.