DL-alpha-tocopheryl acetate, a synthetic form of vitamin E, is routinely given as a dietary supplement to cattle. In this study we assessed the relative bioavailability of three formulations of DL-alpha-tocopheryl acetate in a kinetic study of plasma alpha-tocopherol in four Italian Friesian dairy cows, following intraruminal administration of a gelatin capsule containing 5,000 IU of DL-alpha-tocopheryl acetate. A Latin square design was used so that each animal received all formulations: (A) adsorbed on silica, (M) microencapsulated and (O) in oil form; 5,000 IU of DL-alpha-tocopheryl acetate was also administered intraperitoneally. The treatments were given following a 2-week period on a diet having no vitamin E supplementation with an interval of 8 days between each administration. Blood samples were collected at 0, 1, 10, 11, 21, 30, 48, 72, 96 and 168 h after each administration. The mean initial plasma alpha-tocopherol concentration (CO) was 2.38 +/- 0.57 micrograms/mL. Maximum plasma concentrations (Cmax) of alpha-tocopherol, adjusted for pretreatment values, were 3.90 +/- 0.13, 3.29 +/- 0.13 and 4.07 +/- 0.19 micrograms/mL, following administration of the A, M and O forms, respectively. The length of time required to obtain the maximum concentration (Tmax) in plasma was 57.5 +/- 7.8, 76.8 +/- 8.9 and 73.1 +/- 14.1 h, and the area under the curve (AUC) was 503.3 +/- 63, 620.25 +/- 108.5 and 465.4 +/- 38.7 micrograms.h/mL for A, M and O forms, respectively. Administration significantly increased the plasma alpha-tocopherol levels in all cases; however the A and M formulations had a lower elimination rate than the O form.