Time course changes in plasma creatine kinase activity during repetitive physical work were studied. Study groups consisted of a control group who performed sedentary administrative work and an experimental group who performed repetitive physical work in a biscuit factory. Venous blood samples were collected on a Monday prior to work and following work on Monday, Tuesday, Wednesday and Thursday and assayed for plasma creatine kinase. A rise in plasma creatine kinase was observed over the four working days and this rise was significantly greater for the experimental group. Despite this rise, creatine kinase values remained within acceptable limits for both groups. These results suggest that mild, repetitive physical work provides sufficient stimulus for creatine kinase release from skeletal muscle. The mechanism underlying the release of creatine kinase cannot be determined from the present study, but it is unlikely that muscle damage was the cause. It is proposed that increased plasma creatine kinase following mild occupational work may be related to increased rates of muscle turnover, stimulated by muscle use, and not be indicative of pathological processes associated with muscular strain and fatigue.