A comparison of the effects of replacing L -glutamine with either glycyl-L-glutamine or alanyl-L-glutamine in a KSOM-type medium on the development of mouse preimplantation embryos in vitro has been made. Alanyl-L-glutamine has no significant effect on the rates of blastocyst formation, onset or completion of hatching, and on the numbers of inner cell mass and trophectoderm cells that develop. Glycyl-L-glutamine has no effect on the rate of blastocyst formation; it stimulates slightly the onset of hatching, but significantly increases the numbers of inner cell mass and trophectoderm cells that develop. Embryo transfer experiments comparing media containing either glutamine or glycyl-L-glutamine have not produced any gross abnormal fetal development. Recently, alanyl-L-glutamine has been used to replace glutamine in media for the culture of human preimplantation embryos. The results in this paper suggest that glycyl-L-glutamine may be a better choice of dipeptide.