Studies of enzyme activities in preimplantation mouse embryos of the Swiss-Webster strain have demonstrated several developmental patterns of change [ 1,2] . Among these is the pattern manifested by lactate dehydrogenase (EC.1 .l .1.27) and glucose 6phosphate dehydrogenase (EC.1 .l .1.49). The activities of these enzymes remain constant for the first two days of development, up to the 8-16 cell stage. There is then an exponential decline in activity over the next two days, and the activities in the late blastocysts are only about 1520% of those in the early stages of development. The cause of the decline in activity is not known, but one possibility is the existence of specific processes leading to enzyme degradation or inactivation [l] . However, Gibson and Masters have recently suggested, on the basis of work with the Quackenbush strain, that the high activity of lactate dehydrogenase and the developmental decline in activity detected in mouse ova do not reflect the true enzymatic situation of the embryo [3]. Rather, they proposed that virtually all of the activity detected is the product of enzyme adsorbed from the oviducal fluid, and the decline in activity is attributed to loss of this surface enzyme secondary to changes in the surface and location of the embryo. Because of the obvious importance of these suggestions for an understanding of the regulation of enzyme concentrations in early mammalian embryos, the activity of lactate dehydrogenase in embryos from Swiss mice has been reinvestigated. The results obtained are not compatible with the hypothesis that the enzyme being assayed is adsorbed from the oviducal fluid. Mouse embryos ranging from 2 cells (day 1) to the late blastocysts stage (day 4) were obtained from random bred Swiss mice by the methods previously described [l] All flushing of the oviducts and handling of the embryos was carried out in the culture medium of Whitten and Biggers [II]. Follicular oocytes were obtained by pricking follicles of ovaries removed two days after the unmated animals received the priming dose of pregnant mare serum. Embryo culture and enzyme assay were carried out as previously described [ 1,2] . Electrophoresis in 7.5% polyacrylamide gels was carried out at pH 8.1 with a continuous tris-glytine buffer system. The stacking gel was omitted. The gels were stained for lactate dehydrogenase activity with a mixture consisting of 1.6 g Na-D, L-lactate, 10 mg NAD, 0.8 mg phenazine methosulfate, and 10 mg nitro blue tetrazolium in 40 ml of 0.05 M trisHCl, pH 7.1.