The aim of this study was to evaluate the effects of a water in oil (w/o) emulsion of the gonadotropin-releasing hormone agonist buserelin ([D-Ser(tBu)(6), Pro(9)-NEt]-GnR-Ha) in Freund's incomplete adjuvant (GnRHa-FIA) on the induction and synchronisation of ovulation in an early spawning strain (late August-September) of rainbow trout, Oncorhynchus mykiss. Mature females were injected once with 0, 25, 50 mug kg(-1) of GnRHa-FIA or 25 mug kg(-1) of GnRHa in physiological saline (acute treatment). All fish dosed with 50 mug kg-1 of GnRHa-FIA ovulated within 10 days after injection, whereas fish dosed with 25 mug kg(-1) of GnRHa-FIA reached 100% ovulation 13 days after injection (water temperature, 11.5 degreesC). In contrast, ovulation percentage was 71% in control group, but it was only 66% in fish treated with acute injection of GnRHa 47 days after injection. Fertilization, eyeing and hatching percentages were high in all treatment groups and did not differ significantly among groups (p>0.05). None of the treatments caused any prespawning mortality. However, post-spawning mortality was observed in female rainbow trout treated with the low and high dose of GnRHa-FIA (46% and 53%, respectively). In conclusion, both GnRHa-FIA doses were effective in advancing the onset of ovulation, synchronising the ovulation of the treated fish and shortening the reproductive period, relative to the control or GnRHa injected group. GnRHa-FIA treatments did not affect oocyte diameter or egg quality based on the fertilization, eyeing and hatching percentages. There was, however, a very significant post-spawning mortality, reaching 50% of the population. (C) 2004 Elsevier B.V. All rights reserved.