Rat liver enzymes that normally accumulate in the third postnatal week did so 1 or 2 weeks earlier upon the administration of different stimuli. Thyroxine was an effective stimulus for malate: NADP oxidoreductase (EC 184.108.40.206) and pyruvate kinase (EC 220.127.116.11), but was without effect on glucokinase (EC 18.104.22.168). Pyruvate kinase and ATP citrate lyase (EC 22.214.171.124) responded to intragastric glycerol, glucose or fructose, but malate: NADP oxidoreductase remained undetectable. Prematurely established high glucokinase levels were not followed by premature accumulation of ATP citrate lyase, pyruvate kinase or malate: NADP oxidoreductase in the same livers. Estradiol treatment which inhibits the normal developmental accumulation of glucokinase did not affect that of the other three enzymes. The observations suggest that enzymic differentiation in liver during the late sucking period is controlled by a concerted action of several physiological stimuli. It is possible to resolve this complex process by matching each stimulus with the subcluster of enzymes whose formation it promotes.