Of 275 mares receiving prostaglandin F2alpha-Tham salt for its luteolytic effect upon the corpus luteum, 231 (84 per cent) exhibited signs of oestrus (range 73-95 per cent). Some mares not exhibiting overt oestrus may, nevertheless, ovulate and post-treatment rectal palpation of mares apparently failing to respond is suggested in these cases. Of 210 mares for which subsequent history was available. 104 (49.5 per cent) became pregnant from breeding at the post-treatment oestrus that immediately followed the induced regression of the corpus luteum (range 42.2-55.5 per cent). Fertility is comparable at this oestrus to that of naturally occurring oestrus in Thoroughbred mares. Side effects with PGF2alpha-Tham are physiological responses to the prostaglandin and include various degrees of sweating, some hypermotility of the gastrointestinal tract, increased pulse and respiratory rates and, occasionally, abdominal discomfort. These signs only occur in a proportion of mares, are transitory in nature and are resolved without treatment. A brief review of a comprehensive toxicity study using 20-160 times the recommended dose level, daily for 8 days, concludes that PGF2alpha-Tham has a 20 times safety margin for a single injection.