The implication of thoracic duct Iymph in the distribution and the elimination of rabbit muscle creatine phpsphokinase (CPK) was investigated in situ and in vitro with 30 rabbits. The rate of thoracic duct lymph flow per kg body weight in anesthetized rabbits was 0.97+-0.21ml.h-1.kg-1 (mean +- S.D., n=9). The CPK activity (U) that appeared in thoracic duct lymph after laparotomy (15 cm dissection along the mid-line) was 0.129 +- 0.038 U/h (n = 4) in untreated rabbits and 4.55 +- 2.23 U/h (n=5) in rabbits given intramuscular administration of CPK(1000U/kg body weight). The rate of transfer of CPK from the circulation to the thoracic duct lymph in rabbits after intravenous administration of CPK (1000 U/kg) was 30.0 +- 11.1 U/h (n=5).The inactivation rate constant of CPK in thoracic duct lymph (pH 7.40, 39℃,0.098 +- 0.023 h-1, n= 5) was larger than that in heparinized whole blood. The result implies that CPK may be partly inactivated in the lymph in vivo. The inactivation rate constant of CPK in thoracic duct lymph fluid without lymphocytes (pH 7.40, 0.159 +-0.013h-1, n=3) was much larger than that in the thoracic duct lymph. The calcium conceotration (14.4 +- 0.8l0mg/dl, n=7)and the magnesium concentration (3.93 +- 0.530mg/dl, n=7) in thoracic duct lymph were determined. The possible mechanism involved in the inactivation of CPK in thoracic duct lymph is discussed.