The protein content of cattle oocytes and preimplantation embryos produced in vivo, from the two-cell to the elongated blastocyst at day 16, was determined. From the oocyte to the expanded blastocyst stage (day 8), protein determination was carried out on zona pellucida-enclosed embryos. Protein content was measured by the Pierce Micro BCA protein assay. The mean protein content of oocytes was 0.126 mu g, with no significant increase at the two-cell stage (0.132 mu g). Protein content was higher at the morula stage (0.183 mu g; P < 0.05) with a further increase at the expanded blastocyst stage (0.367 mu g; P < 0.05). There was a 160-fold increase in protein content from the expanded blastocyst to the hatched day 13 stage. Spherical, ovoid and elongated blastocysts were collected on days IS and 14. The mean protein content of day 13 (59.8 mu g) and day 14 (92.4 mu g) embryos was similar (P > 0.1), but the protein content of the elongated embryos was higher than that-of ovoid or spherical embryos collected on the same day. Protein content of day 15 embryos (362.2 mu g) was higher than that on day 14, with a further increase to 946.6 mu g by day 16. The correlation between protein content and day of development contained both a linear and a quadratic component. Embryo length and width increased from day 13 (5.24 mm and 0.89 mm, respectively) to day 16 (51.6 mm and 1.82 mm, respectively). From day 13 to day 16, the protein content was correlated with both embryo length and width (r(2) = 0.89 and 0.51, respectively; P < 0.001) and was highly correlated (r(2) = 0.95) with the product of embryo length by width, indicating that protein content increases as a function of surface area.