In the first of two experiments, numbers of ovulations and day 5 embryo yields were unaffected by fish oil (0, 3 or 6% w/w) in ewe diets; oestradiol concentrations and numbers of large ovarian follicles were increased. Fish oil tended to suppress embryo development and limit blastocyst diameter and cell number but not protein synthesis. In the second experiment IVF-derived embryos were cultured in medium supplemented with foetal calf serum (FCS) or, as in vitro equivalents to Experiment 1, serum from ewes fed rations supplemented with 0 (C), 3 (3F) or 6 (6F)% fish oil. Cleavage rates were not affected by serum source. More Grade 1 to 1.5 blastocysts were produced when using 3F serum (32% of blastocysts) than with C (27%; ns) or 6F (16%; P<0.05) or FCS (10%; P<0.005). It is suggested that at less than 3% inclusion in the diet of ewes, there may be beneficial effects of fish oil fatty acids on embryo production in vivo and of the corresponding ewe serum during in vitro culture. Higher concentrations are counter-productive.