The present study was designed to investigate the endocrinological and morphological events which occurred in human granulosa cells. The results were as follows: Human granulosa cells harvested from preovulatory large follicles secreted 0.55 approximately 1.01 pg/cell/day of progesterone(P). During 7 days of culturing, the total P productive level was 3.96 +/- 0.22 pg/cell (mean +/- SE). It level per cell per 24 hours was the highest on the third day, after which the level tended to decrease. The granulosa cells obtained from the preovulatory large follicles secreted 2.12 +/- 0.41 pg/cell (mean +/- SE) of estradiol (E2) during 6 days culturing E2 productive volume per cell measured every 48 hours however tended to decrease with increasing time in the culture. The fine structure of the cultured granulosa cells had almost the same appearance on the third day as on the seventh day of the culture. Namely, they displayed numerous lipid droplets, widely dispersed Golgi complexes and mitochondria with plain cristae. However, smooth endoplasmic reticulum were only seen sporadically. Judging from the cultured results of granulosa cells, they showed an early stage of formation of corpus luteum in the fine structure which was assumed to be reflected by the endocrinological events which secreted P and E2.