Inhibins and activins are firmly implicated in the control of pituitary FSH secretion and ovarian follicular development in mammals. As in mammals, inhibin A and activin A are expressed in the preovulatory follicles of birds, and a defined ovulation cycle for inhibin A has recently been demonstrated in the laying hen. To investigate further the role of inhibin-related proteins in developing pullets, circulating concentrations of inhibin A, inhibin B, total immunoreactive inhibin α-subunit (ir-α), activin A, LH, FSH, and progesterone were measured from the juvenile state through to sexual maturity in 22 birds. In the 11 birds assigned to control groups, plasma inhibin A levels were low from 7 to 13 wk of age rising about threefold to a peak at Week 19 after which levels fell slightly to a plateau level characteristic of adult hens. Plasma inhibin A levels were negatively correlated with FSH (r = −0.33; P < 0.001) and positively correlated with progesterone (r = 0.67; P < 0.001) and ir-α (r = 0.53; P < 0.001). Plasma ir-α levels were much higher than inhibin A levels although the relative differences varied with age. Plasma levels of inhibin B and activin A were below assay detection limits at all times. The remaining group of 11 birds was actively immunized (IMM) against a synthetic chicken inhibin α-subunit peptide (amino acids 1–26). The IMM generated circulating antibodies that bound native bovine inhibin A but altered neither plasma FSH nor progesterone levels relative to control birds at any stage of development nor the timing of first oviposition in week 19. Apart from a transient decline 1 wk after primary IMM, plasma LH concentrations did not differ from controls. Comparison of the numbers and size-class distribution of ovarian follicles at 29 wk showed an approximate twofold increase in the number of 8- to 9.9-mm-diameter follicles (control; 1.82 ± 0.44 vs. IMM; 3.91 ± 0.89; P < 0.05), a size class that corresponds to follicles that have just joined the preovulatory hierarchy. The numbers of growing follicles in other size-classes and the sizes of hierarchical F₁–F₇ follicles were not altered by IMM. However, the number of postovulatory follicles increased (control 3.73 ± 0.20 vs. IMM 5.55 ± 0.28; P < 0.01), and significantly more (P < 0.02) immunized hens laid two eggs within a 24-h period on at least one occasion (control 1 of 11 vs. IMM 9 of 11). The IMM increased (P < 0.05) activin A content of F₁ and F₂ theca layers and decreased (P < 0.05) activin A content in F₃ and F₄ granulosa layers, raising the possibility of a local intraovarian role of activin in mediating the response to IMM. These findings support a role for inhibin A in regulating the entry of follicles into the preovulatory hierarchy in the chicken, although further studies are required to establish the mechanism by which inhibin IMM increases the rate of follicle selection and ovulation without raising plasma FSH.