Pouch young were removed from 10 wallabies in lactational diapause, and half the animals were injected intravenously with 8 ml normal wallaby serum or 8 ml GnRH antiserum every 3rd day from the day of removal of pouch young until Day 30. Laparotomies were performed on Day 24 to monitor luteal size, follicular development and uterine enlargement. The pouches were examined daily for the presence of newborn young from Days 26 to 30, and all the animals were killed on Day 30. The corpora lutea had hypertrophied in all the animals of both groups by Day 24, but none of the antiserum-treated animals showed any signs of follicular development (follicles less than 1 mm diam.), whereas all the control animals had large follicles (mean 3.5 mm diam.). In each group 4/5 animals were visibly pregnant at laparotomy, and parturition occurred normally in 3 of the experimental animals and 1 of the controls. At autopsy, none of the antiserum-treated animals showed any evidence of follicular development or post-partum ovulation, whereas 3 of the controls had new corpora lutea and the other 2 had large preovulatory follicles. These differences were reflected in the weights of the lateral vaginae; the treated animals showed no evidence of oestrogenic stimulation (4.9 +/- 0.2 g), whereas the controls showed the hypertrophy characteristic of oestrus (9.5 +/- 1.2 g). Lactogenesis, as measured by an increase in mammary gland lactose concentrations, was established in all animals, regardless of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)