Protein kinase C (PKC) may be involved in growth regulation. In the present study the relationship between body weight, and thereby age, and the activity of PKC in muscle as well as in rapidly growing overloaded muscle were investigated. PKC activity in music was linearly inversely correlated to rat weight in both soleus (r = -0.59, P less than 0.05) and in plantaris (r = -0.74, P less than 0.01) muscles. During compensatory hypertrophy. PKC activity per muscle was maximally increased compared with the contralateral control muscles after 4 days in both soleus (126%) and in plantaris (105%) but had returned to basal levels by the 9th day. The data are in agreement with a role for PKC in muscle growth.