Muscles from stress-susceptible (SS) pigs are often characterized by a rapid rate of glycogenolysis, but the mechanism which is responsible for this abnormality is unknown. As part of an effort to define the mechanism, we determined the relationship between cyclic AMP (c-AMP) and phosphorylase a activity in SS and control (C) pigs. Longissimus muscles were obtained from SS and C uias at 3. 30. 60 and 120 min postexsanguination. At 3 mm, c-AMP levels were 1555 ? 94 and 1309 ± 29 pica moles/g muscle from SS and C, respectively, and differed significantly (P = 0.05). Levels at ofher times were not significantly different. Ratios of phosphorylase a to a±b (total) activities at 3 min were 0.303 ± 0.047 and 0.157 ± 0.028 for SS and C, respectively, and differed significantly (P = 0.05). Differences were not significant after 3 min. Total activities at 3 min were 0.617 ± 0.119 and 0.535 ± 0.047 mg phosphorus liberated/10 min/mg protein. These results, without accounting for ofher mechanisms which might regulate phosphorylase activation, indicate that higher c-AMP levels at 3 min could produce higher phosphorylase a levels in SS than in C pigs. Whether this difference is adequate to explain the difference in rate of glycogenolysis needs further clarification.