Observations on the repeated use of gonadotropin treatments in the dairy goat. Our studies have attempted to determine if the absence of oestrus observed in some goats after fluorogestone acetate-pregnant mare serum gonadotropin (FGA-PMSG) treatments could be due to its repeated use. An experiment was carried out in a Saanen flock, in which she-goats were submitted to FGA-PMSG treatment each year. The first treatment of the year was performed on 169 shegoats between March and August 1989. Of the females diagnosed as non-pregnant following the first treatment, 38 were treated a second time during the same period. The proportion of females which went into oestrus after the first treatment was lower in 59 multiparous goats than in 64 nulliparous and in 46 primiparous goats (64.4 vs 100 and 97.8%; P < O.Oi). For the she-goats treated twice, the percentage of females which went into oestrus after the second treatment was lower than that observed after the first treatment (44.7 vs 71.0%; P < 0.05). The presence of anti-PMSG antibodies in plasma was investigated in blood samples taken before and after each treatment. Results are expressed as % of radioactive PMSG bound by 10 pl of blood plasma. Before the first treatment, the percentage of bound PMSG was higher in multiparous than in nulliparous and primiparous goats (17.5 t 23.1 vs -0.06 t 0.7 ; 1.2 ± 1.9 ; P < 0.01), and increased after treatment for all parities. In goats treated twice, the percentage of bound PMSG before the second treatment was higher than that observed before first treatment (22.8 t 23. vs 11.2 f 19.7; P < 0.05). For both treatments, females which did not come into oestrus showed a percentage of bound PMSG which was significantly higher than that of goats in which oestrus was observed (multiparous first treatment: 30.5 f 23.6 vs 1I 2. 9 20. 1 %; she-goats in second treatment: 33.9 ± 23.1 vs 9.2 ± 14.5T.). When the percentage of bound PMSG before treatment was low (< 5°l) in multiparous goats, the oestrus and kidding rates after the first treatment did not differ from those observed in nulliparous and primiparous goats. The decrease in the efficiency of the treatments was therefore not due to the age of animals. The repeated use of FGA-PMSG during the lifetime of goats or within the same year is followed by an increase in the levels of antibodies against PMSG. The presence of those antibodies may explain the decrease in the efficiency of these treatments in inducing and synchronizing oestrus.