The effects of a GnRH antagonist analogue (N-acetyl-Ala1,D-p-Cl-Phe2,D-Trp3,6-GnRH, Ant.) and a GnRH antiserum (A/S) on the development of pituitary-testicular function were studied in immature (23/24-31/32-day-old) rats. In another experiment the Ant. treatment was combined with bromocriptine (BR)-induced hypoprolactinaemia. Ant. and A/S decreased serum and pituitary levels of LH and FSH, and BR those of Prl (P less than 0.01-0.05). Testicular testosterone (T) and progesterone (P) contents were significantly decreased only by Ant. (P less than 0.01). Ant. decreased the weights of the testes, ventral prostates and seminal vesicles, as well as testicular LH, FSH and Prl receptors (R) (P less than 0.01-0.05). BR decreased LH-R but had no effect on Prl-R. Both Ant. and A/S decreased available pituitary GnRH-R (P less than 0.01), but free testicular GnRH-R were reduced only by Ant. BR increased GnRH receptors in the pituitaries. It is concluded that Ant.-induced low gonadotropin levels in immature animals inhibit the developmental increase of testicular weight, gonadotropin and Prl-R, steroidogenesis and androgen action on accessory sex glands. Hypoprolactinaemia had an additive inhibitory effect to the antigonadal effects of Ant. The testis tissue of immature (23/24-day-old) animals already contains GnRH-R. In general, developing animals are clearly very sensitive to the antigonadal actions of Ant. and BR, whereas the effect of GnRH-A/S is less pronounced than in adults.