AbstractThe relative developmental abilities of hamster 2‐ and 8‐cell embryos in culture were compared using two versions of hamster embryo culture medium (HECM). These media differed primarily in the number of amino acids present, i.e., 20 amino acids in HECM‐1 and four amino acids in HECM‐2. When 2‐cell embryos were cultured for 24 h, the percentages of ⩾4‐cell embryos obtained in both HECM‐1 and HECM‐2 were comparable (≃93%); at the end of 48 h, the proportion of ⩾8‐cell embryos obtained in HECM‐1 (82.5%) was significantly (P⩽ 0.001) more than that obtained in HECM‐2 (67.9%). Interchange of media, after 24 h culture, did not enhance the ability of cultured 2‐cell embryos to become blastocysts. When 8‐cell embryos were cultured for 18 h in HECM‐1 and ‐2, there was no appreciable difference in the proportion of total blastocysts formed (89‐91%). However, there were significantly (P⩽ 0.001) more late blastocysts in HECM‐2 than in HECM‐1 (68.2% vs. 38.4%). Embryo development from 2‐ and 8‐cell stages was compared in media that differed by the presence and absence of phenol red and penicillin‐G. There was no difference in embryo development when these compounds were present or absent. Similarly, the difference in pyruvate concentration between HECM‐1 and ‐2 (0.5 and 0.2 mM, respectively) did not affect embryo development. These results show that a) the presence of four amino acids (Phe, Ile, Met, and Gln as in HECM‐2) was not sufficient for 2‐cell embryo development in culture and additional amino acids (as in HECM‐1) were required; and b) the four amino acids were better than 20 amino acids for obtaining late blastocysts starting from the 8‐cell stage. The low percentage of late blastocysts obtained in HECM‐1 was attributed to the presence of inhibitory amino acids in this formulation. Hence, it is concluded that the optimum medium for