Summary. Oestrus, expected to be followed by a short luteal phase, was induced in post-partum cows by weaning their calves at 35 days after parturition. Ovaries containing the first preovulatory follicles (Type F) formed after parturition were collected 3 h after the onset of oestrus. For comparison, preovulatory follicles (Type C) were collected 3 h after the onset of oestrus in normally cycling cows. The number of granulosa cells was determined and the concentrations of receptors for follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in granulosa cells and for LH in theca cells were measured. Concentrations of oestradiol-17\g=b\,testosterone, androstenedione and progesterone in follicular fluid were also measured. Type F follicles contained about twice the number of granulosa cells (based on DNA) as did Type C follicles (45\m=.\8\m=+-\ 1 1 \ m= . \ 3 and 24\m=.\5 \ m=+-\ 3\m=.\9 \g=m\gDNA/follicle, respectively; P < 0\m=.\05) but these cells had fewer receptors for LH (0\m=.\13\ m=+-\0\m=.\02vs 0\m=.\29\ m=+-\ 0\m=.\03fmol/\g=m\g DNA; P < 0\m=.\01) and FSH (0\m=.\61\ m=+-\0\m=.\08vs 1\m=.\3\ m=+-\0\m=.\29fmol/\g=m\gDNA; P < 0\m=.\08) than did those from Type C follicles. Additionally, there were fewer receptors for LH in theca tissue from Type F than from Type C follicles (28\m=.\3 \m=+-\5\m=.\2vs 51\m=.\3\ m=+-\6\m=.\1 fmol/follicle; P < 0\m=.\01). Concentrations of oestradiol-17\g=b\(475\m=.\8\ m=+-\ 85\m=.\6vs 112\m=.\9\ m=+-\ 40\m=.\0 ng/ml; P < 0\m=.\01) and androstenedione (214\m=.\1 \m=+-\48\m=.\7vs 24\m=.\7 \ m=+-\7\m=.\7ng/ml; P < 0\m=.\01) in follicular fluid were higher in Type C than in Type F follicles. There were therefore more granulosa cells in Type F follicles which contained fewer receptors for LH and FSH and accumulated less oestradiol-17\g=b\ in the follicular fluid. We suggest that the characteristics of preovulatory follicles may influence the life-span of the subsequently formed corpus luteum and short luteal phases may de due, in part, to altered follicular development.