Recent experimental evidence has suggested that the control of ovarian folliculogenesis and ovulation rate in cattle cannot be accounted for solely through changes in pituitary gonadotrophins, gonadal steroids and/or proteins. The aim of this project was to investigate the role of growth hormone (GH), insulin-like growth factor-I (IGF-I) and insulin in the control of ovarian follicular growth and development in heifers. Daily treatment of heifers with 25mg recombinant bovine somatotropin (BST), for a period of 2 oestrous cycles, significantly increased the population of small ovarian follicles (2-5mm in diameter) although ovulation rate ws unaltered. While peripheral concentrations of GH, IGF-I and insulin were significantly increased, there was no effect of BST on either circulating concentrations of oestradiol, progesterone, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) or number of FSH and LH binding sites in granulosa and thecal cells. A further study using real-time ultrasound demonstrated that in our population of heifers the majority (9/12) have 3 waves of follicular growth and development per oestrous cycle, whilst the remainder displayed a 2-wave pattern. The growth of each dominant follicle was always associated with a marked reduction in the number and growth of the subordinate follicles. While there was no effect of BST on both the number of follicular waves and the number of large (> 10mm) and medium-sized (5-10mm) follicles, BST treatment significantly increased the number of small follicles throughout the treatment period. However, the inhibitory effect of the dominant follicle on subordinate follicles was not affected by BST treatment. Treatment of heifers with a single dose of BST (320mg in a sustained release formulation) demonstrated that the increase in the number of small follicles was temporally correlated with changes in peripheral IGF-I and insulin concentrations.