Summary Plasma progesterone concentrations were measured in 179 mares bled on alternate days commencing with a positive pregnancy diagnosis on Days 17 to 18 after ovulation and concluding on Days 42 to 45. During this period 17 mares (10 per cent) lost their pregnancies, 11 before Day 25. In 15 mares the timing of the pregnancy loss could be determined with adequate accuracy; in only one did a decline in progesterone precede the loss. Thus pregnancy loss between Days 17 and 42 was rarely caused by a fall in plasma progesterone.

The possibility of seasonal variation in the feedback effect of testosterone or oestradiol was investigated by giving replacement treatment to geldings for 2-3 weeks during breeding and non-breeding seasons. In the non-breeding season, testosterone suppressed LH values (mean +/- s.e.m., ng/ml) in all geldings (before treatment, 7.5 +/- 2.3; final treatment week, 1.8 +/- 0.2; P less than 0.05), whereas early in the breeding season, testosterone caused a prolonged rise in LH (before, 6.8 +/- 2.3; final week, 18.9 +/- 6.4; P less than 0.05). In all testosterone experiments, LH returned to pretreatment levels within 2 weeks after treatment. Oestradiol treatment caused a prolonged increase (P less than 0.05) in LH concentrations (mean +/- s.e.m., ng/ml) in both seasons (breeding: before 5.2 +/- 1.1; final week, 16.2 +/- 4.8; non-breeding before, 10.9, 20.1 +/- 5.2). We conclude that in geldings the feedback effect of testosterone varies with season and, further, that testosterone replacement may be able to restore to geldings the stallion's seasonal pattern of LH secretion. The results suggest that, in male horses, testosterone and possibly oestradiol, are important components in the neuroendocrine pathway controlling seasonal breeding and, moreover, are essential for the generation of a positive signal for LH secretion in the breeding season.

Summary Our objectives were to determine whether repeated administration of prostaglandin F2α (PGF2α) to simulate the endogenous mode of secretion would be more effective than a single injection in inducing luteolysis and enable use of smaller doses less likely to cause adverse side effects. The main study comprised 43 dioestrous mares, who were given i.m. either a single 10 mg dose of natural PGF2α (n = 22) or 2 doses of 0.5 mg PGF2α 24 h apart (n = 21). The intensity of side effects was assessed in 8 dioestrous mares given 5, 1.5, 0.5 or 0 mg PGF2α in consecutive cycles. Two doses of 0.5 mg PGF2α 24 h apart caused lysis of the corpus luteum in all mares, whether this was determined from a fall in plasma progesterone concentrations or reproductive tract/behavioural changes; and when 10 mg PGF2α was given, the corpus luteum was lysed in 17 of 22 mares i.e. a lower proportion (P= 0.0485). A single dose of 0.5 mg PGF2α was no more effective than saline in inducing luteolysis. The intensity of side effects of PGF2α increased with dose. Although the 0.5 mg dose was no more likely than saline to cause sweating ormuscle spasms, it raised plasma cortisol concentrations and prevented the decline in heart rate seen after saline. We conclude that a 2 dose regimen of administration increases the luteolytic efficacy of PGF2α and thereby provides a way to minimise adverse side effects.

In mares, dioestrous FSH profiles based on once-a-day sampling are variable; however, the pulsatility of plasma FSH, which

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