In the hen ovary, each preovulatory follicle in the hierarchy, irrespective of its size and the level of its maturity is exposed to the preovulatory LH surge in each ovulatory cycle of an egg laying sequence. In the present study, the thecal weight and membrane protein content of theca layers at different stages of hen ovulatory cycle were assessed. Hens were killed 2 h (stage I), 9 h (stage II), 16 h (stage III), and 23 h (stage IV) after oviposition. The first (F1), second (F2), third (F3), fourth (F4) and fifth (F5) largest yellow follicles were utilized. In all follicles except F1, the thecal weight rose considerably between stages I and III (P < 0.05) followed by a slight cessation of the thecal growth at stage IV. The mean content of the theca membrane protein in F1-F5 follicles was lowest at stage III, increasing at stage IV (P < 0.05), although, in the case of individual follicles the difference was significant (P < 0.05) in F3 follicles only. Estradiol-17beta levels in the plasma were lowest (but not significant) at stage III, and a fourfold increase in the plasma progesterone concentration occurred at stage IV. These findings demonstrate for the first time the ovulatory cycle-related alterations in the thecal weight and membrane protein content in the hen preovulatory follicles. Data suggest that the preovulatory rise in ovarian steroid hormones is probably involved in transient termination of the growth and induction of differentiation of the theca in preovulatory follicles as they pass from one category to the next.