An intriguing question in the evolution of the mammalian brain is how pattern can be conserved despite enormous changes in size and functional capacities. This phenomenon is dramatically demonstrated in the striatum where compartmental pattern is conserved across different mammalian species (Johnston et al., 1990)(Fig.1). Although the majority of the mammalian striatum is comprised of a seemingly homogeneous population of medium spiny neurons (Kemp and Powell, 1970), it can be compartmentalized into two subdivisions, the patch and matrix compartments, on the basis of different distributions of neurotransmitters, receptors and connections (Graybiel et al. 1981; Gerfen et al, 1984; van der Kooy et a1., 1987). Compartmentalization of the striatum into small patches in a matrix background is restricted to mammals (Reiner et al., 1989) and has been shown in rodents (Herkenham and Pert, 1981), cats (Graybiel et al., 1981), monkeys (Goldman-Rakic, 1982) and humans (Haber, 1986). Despite the large differences in total striatal size, the ratio of patch/matrix area and the number of individual patches is maintained in different mammalian species (Johnston et al., 1990; Fig. 1). The maintenance of striatal compartmentalization suggests that common developmental mechanisms are involved in establishing this pattern and raises many questions concerning how this is achieved. We hypothesize, first, that cell lineages are committed to becoming members of compartments while still proliferating in the ventricular zone and that these precursors share common properties with the precursors which produce the compartments of the other major portion of the telencephalon, the cerebral cortex. Second, we suggest that the adult striatal pattern of small patches embedded within a matrix background is produced through cellular interactions in the postmitotic region that position compartmentally committed cells.