This experiment was conducted to determine the relationship between 3-methylhistidine (3MH) production and proteinase activity in skeletal muscles of growing barrows. Barrows at 13 wk of age were randomly assigned to either control diet available on an ad libitum basis (21% of ME consisted of protein; control group), control diet fed restricted (pair-fed with barrows in protein-free group; intake-restricted group), or protein-free diet available on an ad libitum basis (protein-free group) for 14 d. During the last 3 d, blood samples were collected for determination of 3MH production rate, which is a measure of myofibrillar protein breakdown. At slaughter, two muscles were taken: masseter (M) and longissimus (L) muscles. The muscle samples were analyzed for calpastatin, mu-calpain, m-calpain, multicatalytic proteinase (MCP), cathepsin B, cathepsins B+L, and cystatins activities. Both muscles were also analyzed for amounts of DNA, RNA, total protein, and myofibrillar and sarcoplasmic proteins. Growth rate (kilograms/day) was influenced by dietary treatments (P < .05). Fractional breakdown rate (FBR, percentage/day) of skeletal muscle, as calculated from 3MH production rate (micromoles.kilogram-1.day-1), was 27% higher for the protein-free group than for the control group. However, no differences in proteinase activities were observed, except for lower MCP activity in the M muscle of the protein-free group than in that of the other groups (P < .05). In the present study, no direct relation was observed between myofibrillar protein degradation rate and proteinase activities in skeletal muscle during a protein-free feeding strategy.