SUMMARY1. Myocardial protein degradation rates were determined by following tyrosine release from rat isolated left hemi‐atria in vitro.2. After two 20 min preincubations the rate of tyrosine release from hemi‐atria was constant for 4 h.3. Skeletal muscle protein degradation was determined by following tyrosine release from rat isolated hemi‐diaphragm (Fulks, Li & Goldberg, 1975).4. Insulin (10‐7M) inhibited tyrosine release from hemi‐atria and hemi‐diaphragm to a similar extent. A 48 h fast increased tyrosine release rate from hemi‐diaphragm and decreased tyrosine release rate from hemi‐atria. Hemi‐diaphragm tyrosine release was inhibited by 15 mmol/1 D‐glucose but a variety of concentrations of D‐glucose (0, 5, 15 mmol/1) had no effect on tyrosine release from hemi‐atria. Five times the normal plasma levels of the branched‐chain amino acids leucine, isoleucine and valine had no effect on tyrosine release from either hemi‐atria or hemi‐diaphragm.