We hypothesized that a diet enriched in alpha-linolenic acid would enhance embryonic development relative to diets enriched in linoleic or saturated fatty acids. Twenty-four lactating Holstein cows (86 +/- 22 d postpartum) were assigned to one of three diets containing saturated fatty acids (SAT; high in palmitic and stearic acids), whole flaxseed (FLX; high in alpha-linolenic acid) or sunflower seed (SUN; high in linoleic acid). Rations were formulated to provide 750 g supplemental fat/cow/d in all dietary groups. Ovulation (Day 0) was synchronized approximately 20 d after diets began. Ultrasound-guided follicular ablation of all follicles >8 mm was performed 5 d after ovulation; superstimulatory treatments began 2 d after follicular ablation, and embryos were collected non-surgically 7 d after AI. Fertilization rate, numbers of follicles and ovulations, and total and transferable embryos did not differ (P > 0.05) among dietary groups. Sixty-one transferable embryos were stained and total blastomere number determined. Blastomere number was affected by diet (P < 0.0 1); without regard to stage of development, embryos collected from cows fed SAT had lower (P < 0.0 1) blastomere numbers (mean S.E.M.; 77.1 +/- 3.9) than those from cows fed FLX (93.4 +/- 3.3) or SUN (97.2 +/- 3.5). Differences were most evident in the expanded blastocyst stage; at this stage, embryos of cows fed FLX and SUN diets had more blastomeres (P < 0.02) than those of cows fed SAT (115.4 +/- 6.3, 132.3 +/- 8.3, and 89.3 +/- 9.6 cells, respectively). Although our hypothesis was only partially supported, embryonic development was enhanced in Holstein cows fed unsaturated fatty acids compared to those fed saturated fatty acids. (C) 2007 Elsevier Inc. All rights reserved.