One hundred-and-two 13- to 18-day blastocysts were collected from 98 Scottish Blackface ewes mated to one of four New Zealand Romney rams heterozygous for the Massey I (t1) translocation. The karyotype of 75 blastocysts was established, of which 22 were 54, XY, 18 were 54, XX, 20 were 53, XY, t1, and 15 were 53, XX, tι. It is suggested that these results indicated that there was no increase in embryonic loss in ewes mated to rams heterozygous for the Massey I (t1 translocation and that the unbalanced secondary spermatocytes in heterozygous males probably did not mature to spermatozoa