Plasma creatine phosphokinase activity (CPK) of hereditarily muscular dystrophic, normal and heterozygous chickens was determined using a method modified from that of Tänzer and Gilvarg. Plasma CPK activity of homozygous dystrophic chickens was significantly higher than that of normal chickens at 27-35 days of age and continued to rise to 131-134 days of age. At ten to 16 months of age plasma CPK activity was lower than at 131-134 days of age, yet still significantly higher than normal. Heterozygotes, sampled at 10 to 16 months of age, revealed significantly higher than normal plasma CPK activity despite great variability with some individual values falling within the normal range.