The activities of five enzymes involved in purine salvage and catabolism--hypoxanthine phosphoribosyl transferase (HPRT), adenine phosphoribosyl transferase (APRT), adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP) and guanase--were measured in mouse embryo extracts, from the one-cell to the blastocyst stage. Xanthine oxidase activity was not detected. The analyses were performed using high performance liquid chromatography and the enzymes showed different patterns of activity during development. Activities of HPRT, APRT and PNP were low before morula formation, and then increased until the blastocyst stage. ADA and guanase showed high activities after fertilization; guanase activity decreased sharply after the two-cell stage and ADA activity decreased sharply after the morula stage. Blastocyst formation was accompanied by a further decline in activity of both enzymes. The methods used may be suitable for measuring these enzymes in single human embryos, or in biopsies derived from them.