A total of 939 one-, two- and four-cell rabbit embryos were cultured at 37 C in defined media containing glucose, salts, amino acids, vitamins, trace elements, and bovine serum albumin (BSA). Dow Corning 360 Medical Fluid was used successfully to cover embryos cultured in individual cups in place of oil previously used to cover droplets of media in culture dishes. The addition of thymidine inhibited development of two- and four-cell embryos into expanding blastocysts, but hypoxanthine had no apparent effect. A level of 1.5% BSA was markedly superior to a 0.15% level in the production of expanding blastocysts (67% vs. 12%). Phenol red at levels of 0.012 µg and 1.2 µg/ml of medium had no effect on embryo development. Starting with one-cell embryos 29% developed to the expanding blastocyst stage in the defined medium containing 1.5% BSA as compared to 64% of the embryos cultured from the two- and four-cell stages. However, 48% of one-cell embryos developed into expanding blastocytes in the optimal treatment containing 0.11 mg/ml of sodium pyruvate. The addition of pyruvate had no effect on the older embryos cultured. When the concentrations of methionine, cysteine, serine, threonine and tyrosine were increased tenfold, embryo development was inferior to results obtained in the basal medium containing amino acids at the levels found in Ham’s F10 medium.