Leydig cell development was investigated in the marmoset (Callithrix jacchus) testis from 24 h post partum (pp) up to sexual maturity, using ultrastructural and histochemical methods. Electron microscopically three different Leydig cell (LC) types could be distinguished: neonatal, immature and adult LCs. Neonatal LCs exhibited a round nucleus, large tubular mitochondria, abundant smooth endoplasmic reticulum (SER), and few small fat vesicles. The typical immature LCs showed a lobulated nucleus, smaller tubulo-vesicular mitochondria, less SER, and larger fat vesicles. Adult LCs contained an invaginated nucleus, tubular mitochondria, abundant SER, large fat vesicles, and lipofuscin granules. Neonatal Leydig cells occurred from 24 h pp up to 10 weeks pp, immature LCs from 2 weeks pp up to 20 weeks pp, and adult LCs from 20 weeks onwards. The appearance of a new LC population was accompanied by a numeric decrease and occurrence of regressive cells of the previous population. Developmental steps of differentiation were recognizable in the population of immature and adult LCs by the amount of SER (immature LCs), the size of mitochondria (adult LCs), and the morphology of the nucleus at the beginning of their appearance. Activity of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) was moderate in immature LCs from 2 weeks pp to 12 weeks pp and strong in adult LCs. Neonatal LCs were not stained nor were immature LCs from 16 weeks to 20 weeks. Comparison of the ultrastructural classification of LCs and the activity of 3 beta-HSD showed that the capacity for steroid biosynthesis does not necessarily include morphological differences. Activity of non-specific esterase (nE) was found in LCs except of the period between 16 and 25 weeks pp. A transient decrease of activity was seen during the first week of life. Both, the phase of decrease and the phase of inactivity were followed by distinct activation of nE. Lipid content and the size of single fat droplets in LCs, as seen with Sudan black staining, varied during postnatal development. Finely stained granules were typical for neonatal LCs, whereas larger droplets were found in immature and adult cells. Particularly high amounts of lipids were seen at one week and between 32-60 weeks pp, which was the regression of neonatal LCs, showing ultrastructurally higher amounts of lipids than intact neonatal LCs. High amounts of lipid during puberty (32-60 weeks pp) accompanied by an activation of nE at the onset of puberty suggest an initiative role in steroidogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)