The dispositions of galactosyl-containing glycoconjugates were studied during postnatal development of the caudate putamen in mice. The binding of the lectin peanut agglutinin, which has an affinity for galactosyl B-1,3 N-acetylgalactosamine residues, was compared to acetylcholinesterase staining and tyrosine hydroxylase immunoreactivity in the immature and adult neostriatum. The binding of peanut agglutinin conjugated to horseradish peroxidase, in sections that were processed for peroxidase histochemistry, was extremely pronounced in the neostriatum through the first postnatal week and constituted ringlike or polygonally shaped structures, which, overall, produced a variegated mosaic. These structures consist of outer rims of dense lectin-associated reaction product surrounding lightly labeled centers. Lectin delineations of the neostriatal mosaic are no longer visible in the second postnatal week. When adjacent sections were processed for lectin binding or acetylcholinesterase histochemistry, the dense lectin binding sites represented borders of acetylcholinesterase-rich and -poor zones. The distribution of dense patches of tyrosine hydroxylase immunoreactive fibers and terminals also coincides with the acetylcholinesterase-rich zones during the same times, and thus the glycoconjugate-delineated boundaries can also be directly compared with the distribution of nigrostriatal dopaminergic projections. The findings presented here represent the first demonstration of a probe that recognizes apparent borders of neostriatal compartments during a limited period of development. They are consistent with previous observations made on transient glycoconjugate "hidden boundaries" during development of other central nervous system structures, including the somatosensory cortical barrel field, and thalamic and brainstem nuclei (Cooper and Steindler, '86a,b; Steindler and Cooper, in press). In those studies, glia were shown to be the major source of glycoconjugate-associated patterns, and thus, glia and glycoconjugates that they synthesize during pattern formation events may be involved in the formation and stabilization of neurochemically distinct components of the neostriatal mosaic.