In Exp. 1, 30 Standardbred mares in deep seasonal anoestrus were divided into 3 equal groups and treated with 0, 50 (G50) or 100 (G100) ng GnRH kg-1h-1 for 28 days via osmotic minipumps. Ovulation occurred in 0/10, 3/10 and 7/10 mares respectively (P less than 0.05). Plasma GnRH profiles (Days -6, 0, 2, 6, 12, 20, 28 and 34 relative to pump insertion) were dose-dependent (P less than 0.01) and peaked on Day 12 of infusion. Mean daily plasma LH concentrations were biphasic in treated mares that ovulated, with LH peaks occurring around Day 6 and Days 16-20. By contrast, in treated mares that did not ovulate the initial LH rise was followed by a steady decline to the end of the experiment. LH pulse frequency in treated mares increased between Day 0 and Day 21 of the experiment. LH pulse frequency in G100 mares was higher (P less than 0.05) than in G50 and control mares on Day 3, and higher than the controls on Days 7 and 21 of the experiment. There were no significant differences in LH pulse amplitude between the groups on the days studied. In Exp. 2, 27 Standardbred mares in shallow seasonal anoestrus received no treatment (N = 13) or a subcutaneous infusion of GnRH (100 ng kg-1h-1) via osmotic minipump for 28 days (N = 14). Mares were served by a stallion during oestrus. Day of ovulation was earlier in treated than in control mares (18.6 +/- 2 vs 41.9 +/- 6 days; P less than 0.001). Likewise, time of conception was earlier in treated than in control mares (25.2 +/- 6 vs 49.1 +/- 9 days; P less than 0.05). One mare in the control group failed to conceive while one treated mare conceived to an undetected ovulation. The results show that constant GnRH infusion induces ovulation and fertile oestrus in mares during deep and shallow seasonal anoestrus.