The inter-relationship between serum and follicular fluid prolactin, oestradiol, progesterone, follicle stimulating hormone (FSH), and luteinizing hormone (LH) in two groups of women was investigated. In group 1, 32 women were treated with gonadotrophin-releasing hormone agonist (GnRH-a) in a long term protocol and subsequently stimulated with human menopausal gonadotrophin (HMG). In group 2, 25 women were simultaneously stimulated with GnRH-a in a short protocol with HMG. Follicular fluid was collected from 54 follicles in group 1 and 47 follicles in group 2. Serum was obtained on the day of human chorionic gonadotrophin (HCG) administration. Serum prolactin and oestradiol concentrations were significantly higher (P < 0.025 and P < 0.01, respectively) in group 1 than in group 2. Serum LH (P < 0.005), FSH (P < 0.01) and progesterone (P < 0.025) were significantly lower in group 1 than in group 2. Follicular fluid prolactin was significantly higher (P < 0.005) in group 1. No differences were found in follicular fluid progesterone and oestradiol. Follicular fluid LH was significantly lower (P < 0.05) in group 1. Serum prolactin correlated positively with oestradiol in both groups (P < 0.005 group 1; P < 0.02 group 2). No significant correlation was found between serum prolactin and LH in group 1. We conclude that prolactin secretion is independent from LH secretion. Hyperprolactinaemia, which is observed in women stimulated with GnRH-a and HMG, is positively associated with increased oestradiol.