The effect of long term administration of T-2 toxin was studied in 6 Trotter mares during the summer and early autumn. After one complete oestrous cycle (Cycle 1) each mare was given 7 mg purified T-2 toxin per os daily (1 mg/ml in ethyl alcohol) beginning on Day 10 after ovulation in Cycle 2. Exposure to toxin was continued for 32-40 days, until Day 7 of Cycle 4. During this period all the animals remained in good physical condition, but skin lesions were observed around the mouth in 3 cases. Toxin administration had no effect on the length of the interovulatory interval or on the lengths of the luteal and follicular phases. It also had no effect on peripheral plasma progesterone profiles or follicular kinetics, including the growth rate and maximum size of ovulatory follicles and the maximum number, and time of first increase in number, of follicles larger than 1 and 2 cm. Uterine flushing was attempted in 5 mares and yielded 3 embryos (60%). These were of normal size and morphology and one was transferred nonsurgically to a recipient mare; a conceptus developed initially but it subsequently died and was resorbed. The results of the study allowed the conclusion that the administration of 7 mg/day T-2 toxin to mares in good body condition has no detrimental effect on ovarian activity, fertilisation or oviductal transport.