Ovaries from normal adult dairy cows were obtained at all days of the estrous cycle. The largest Graafian follicle and corpus luleum were examined for the enzyme succinic dehydrogenase and lipids. Quantitations of the fractional volumes of the enzyme-reactive and lipid-reactive tissues of the total volumes of the follicle wall or corpus luteum w ere made using the linear scanning technique. The staining intensities of the cells were also quantitated. The succinic dehydrogenase reactivity was low in the follicle cells (except during atresia) with an increase during proestrus and estrus in the theca interna cells. In the corpus luteum, the enzyme reactivity was high in the large luteal cclis during late metestrus and diestrus but was low in the small luteal cells. The total lipid reactivity was low in the mcmbrana granulosa cells with a slight increase before ovulation; it was higher in the theca interna cells but with a preovulatory decrease and change to granules of finer type in the epithelioid theca interna cells. The lipid reactivity of the large luteal cells was low and the cells contained fine granules. The small luteal cells, in contrast, had a high lipid reactivity and larger granules, especially during the second half of the estrous cyclc. A cholesterol reaction was observed in the small luteal cells during corpus luteum regression, while weak or no reactions were observed in the follicle or large luteal cells. The observations indicated that strong succinic dehydrogenase and weak lipid reactivities occur during periods of reported maximal steroid synthesis and maximal cell growth. The reactivities reversed during periods of reported minimal steroid synthesis and minimal cell growth.