In the adult rat the striatum is a compartmentalized structure, which is reflected in the inhomogeneous distribution of dopamine. As a first step to test the hypothesis that dopamine plays an organizational role in the development of the striatum, the ontogeny of the dopaminergic system was studied in detail with immunocytochemical methods employing antibodies against dopamine. Rat embryos, fetuses, pups and adults were perfusion-fixed with glutaraldehyde on all prenatal days from E11 onward, postnatally on P2, P4, P6, P7, P8, P13, P14, P20, P21, and in adult age. On E13 the first dopaminergic cells are detected in the ventral prosencephalon. On E14 two dopaminergic cell groups are present in the ventral mesencephalon, and fibres of these cells reach the ventrolateral part of the ganglionic eminence. In the next two days both the cell groups and their projections rapidly increase in size. On E17 the afferent dopaminergic fibres to the striatum become aligned and form huge bundles that are closely associated with the fascicles of the internal capsule. Rostrally, the development of the striatal dopaminergic innervation shows a clear ventrolateral to dorsomedial gradient, whereas more caudally the dopaminergic fibres innervate the striatum from a ventromedial position. The lateral parts of the otherwise compact mesencephalic cell groups consist of loosely arranged cells. From E17 onward these cells become arranged into a dorsal and a ventral group. Just before birth, on E21, the primordia of the dopaminergic cell groups in the substantia nigra pars compacta and pars reticulata can be observed. On E19 several centres with extensive fibre ramifications along the dorsolateral margin of the caudate putamen represent the first signs of the inhomogeneous distribution of dopaminergic fibres in the dorsal striatum seen during the next two weeks. In the following pre- and postnatal days these so-called dopaminergic "patches" also appear more medially. By the third postnatal week most of the patches are no longer detectable, and only the most dorsolaterally located ones, i.e. in the region where they first were detected on E19, remain visible through to the adult stage. Prenatally, no varicosities can be observed in the dopaminergic fibres. The first varicosities appear after birth. Their number increase rapidly during the first and second postnatal weeks and reaches near adult levels on P20. The development of the striatal dopaminergic innervation, and that of the "patches" in particular, is discussed in relation to the development of the mesencephalic dopaminergic cell groups.(ABSTRACT TRUNCATED AT 400 WORDS)