The effect of ACTH administration on plasma cortisol concentrations in purebred and crossbred pigs was investigated. Pigs were given either 25 IU of ACTH or physiologic saline solution IM. Blood samples were collected immediately before and 1 hour after ACTH or saline solution administration. Administration of ACTH resulted in a significant (P less than 0.01) increase in plasma cortisol concentration compared with that resulting from administration of saline solution; mean values after ACTH administration were similar in both breed groups. In contrast, a fivefold range of differences was observed among individual pigs of the same age, sex, and body weight, irrespective of breed group. The type and magnitude of the adrenocortical response was consistent and repeatable in pigs over a 3-month period, suggesting that pigs have a consistent capacity for adrenocortical response to ACTH administration. Development of a dynamic test allowed the high and low responding extremes in a population to be detected. The most suitable dose of synthetic ACTH was established to be 50 IU, and the best time for blood sample collection was 60 minutes after ACTH administration. The classification of individual pigs as high or low responders was repeatable and was not affected by prior short-term exposure to ACTH.