The testicular gland is the main androgen producing organ in male animals. The production of androgens is regulated by LH, secreted by the pituitary. The experimental work described in this thesis concerns the role of the rate-limiting step in overall testosterone formation in rat testis, that is the rnitochondrially localized production of pregnenolone. In order to properly characterize cellular and subcellular fractions used in this study, the presence and distribution of specific marker enzymes had to be established in these fractions. Carboxyl esterase has been used to characterize testicular interstitial tissue. Cytochrome £ oxidase and monoamine oxidase have been applied to identify mitochondrial fractions, whereas carboxyl esterase, rotenone-insensitive NADPH-cytochrome £ reductase and steroid sulfatase have been used as marker enzymes for microsomal fractions. Glucose-6-phosphate dehydrogenase and lactate dehydrogenase were assayed as reference enzymes for the particle-free supernatant fraction. In appendix paper I, however, it has been suggested that a small part of lactate dehydrogenase activity is localized in mitochondria of specific cell types in the seminiferous tubules.