The objective of this study was to investigate changes in expression of mRNAs encoding FSH receptor (FSHr), LH receptor (LHr), cytochrome P450 side-chain cleavage (P450(scc)), cytochrome P450 17alpha-hydroxylase (P450(c17)), and cytochrome P450 aromatase (P450(arom)) during recruitment and selection of bovine ovarian follicles. Dairy heifers (4-5 per group) were ovariectomized at 12, 24, 36, 48, 60, 72, 84, or 96 h after initiation of the first follicular wave following estrus as determined by ultrasonography (Time 0 = initiation of follicular wave; mean +/- SEM = 42.0 +/- 2.6 h after estrus). Expression of mRNAs encoding FSHr, LHr, P450(scc), P450(c17), and P450(arom) was detected by in situ hybridization and quantified by image analysis. Antral follicles were classified as healthy or atretic. Healthy follicles expressed higher (p < 0.01) amounts of mRNAs for gonadotropin receptors and steroidogenic enzymes than did atretic follicles, and expression of LHr, FSHr, P450(scc), P450(c17), and P450(arom) increased (p < 0.01) with follicular size and stage of the follicular wave. Expression of mRNAs for P450(scc), P450(arom), and LHr was time- and size-dependent during recruitment and selection. During recruitment, expression of mRNAs for P450(scc) and P450(arom) was first detected in granulosa cells of 16 of 21 of the follicles 4-6 mm in diameter at 12 h. At 24 and 36 h, almost all follicles 6-9 mm in diameter, but not those 4-5 mm in diameter, expressed both P450(scc) and P450(arom) mRNA in the granulosa cells. At 48 h and thereafter, P450(scc) and P450(arom) mRNA were expressed predominantly in one healthy large follicle per cow with a few exceptions. Expression of LHr mRNA was first detected in granulosa cells at 36 h and was always found in granulosa cells of one follicle > or = 8 mm per cow with exception of one cow at 36 h (no expression) and another two cows, one each at 36 and at 84 h (expression in 2 follicles). In addition, LHr mRNA expression in the granulosa cell layer was limited to follicles that also expressed mRNAs for P450(scc) and P450(arom) in the granulosa cells. In summary, follicular recruitment in cattle was associated with expression of P450(scc) and P450(arom) mRNA within granulosa cells, and the process of follicular selection was associated with initiation of LHr mRNA expression in granulosa cells.