Serum cortisol and adrenocorticotropin (ACTH) levels were measured on nine sleeping volunteers at midnight, before and after the stress of being woken and exposed to a single standard venepuncture. In six volunteers, plasma ACTH levels were raised within 90 seconds of venepuncture, while it remained undetectable in the other three. Cortisol levels began to rise within 4 minutes. ACTH and cortisol levels may be greatly altered by the stress of venesection. Plasma ACTH measured after a difficult venesection in a subject with poor venous access may be misleading.