Thirty-five non-gestating multiparous Holstein cows averaging 571 kg of BW (SE = 8) were allotted at 9 wk postpartum to one of two dietary fat supplements based on either Megalac® (Volac Ltd., Roston, Hertfordshire, UK) and solvent extracted flaxseed meal (MEGA) or whole flaxseed treated with formaldehyde (FLAX) to determine the effects on milk production and composition, follicular development, gestation rate, and fatty acid (FA) composition of blood. Cows were fed a total mixed diet based on ryegrass silage and fat supplements for ad libitum intake. The experiment was carried out between weeks 9 and 19 of lactation. Dry matter (DM) intake and change in body weight were similar for cows fed MEGA and FLAX. Milk production was higher for cows fed MEGA than for those fed FLAX (19.8 vs. 18.6 kg d–1) as was 4% fat-corrected milk yield (22.9 vs. 20.2 kg d–1). Increased fat mobilization could have contributed to increased milk yield when cows were fed MEGA compared with when they were fed FLAX as plasma concentrations of non-esterified FA and cholesterol increased more from weeks 9 to 19 of lactation for cows fed MEGA. Milk fat percentage tended (P = 0.06) to be greater for cows fed MEGA (4.62%) compared with those fed FLAX (4.37%). Milk protein percentage was higher for cows fed FLAX (3.09%) than for those fed MEGA (2.95%), indicating that formaldehyde protection of flaxseed was adequate to partly prevent ruminal degradability of protein in the seed. Milk fatty acid concentrations of C8:0, C10:0, C12:0, C14:0, C14:1, C18:0, C18:3, and C20:5 were higher for cows fed FLAX than for those fed MEGA while the inverse was observed for C16:0, C16:1, C18:1, and C18:2. Cows fed FLAX had lower blood concentrations of C16:0 than those fed MEGA. There was a significant interaction (P <0.05) between week and diet for C18:0 and C18:2 with a decrease in C18:0 blood concentration for cows fed MEGA and an increase for those fed FLAX between weeks 9 and 19, while the inverse was observed for C18:2. Blood concentrations of C18:1 were similar for both treatments. Conception rate was significantly lower for cows fed MEGA (50.0%) compared to those fed FLAX (87.5%). Diet had no effect on the size of the largest and second largest follicles, or on the difference between the diameter of the largest and second largest follicles. The number of class 1 (1.09 vs. 0.86), 2 (1.33 vs. 0.86), and 3 (1.28 vs. 0.98) follicles was similar for MEGA and FLAX although the total number (3.70 vs. 2.70) of follicles tended (P = 0.09) to be greater for cows fed MEGA than for those fed FLAX. These data suggest that dietary FA have an effect on gestation rate, but this could not be explained by differences in follicle dynamics or number. However, additional trials with greater numbers of animals are needed to confirm the reproductive results.