Psychological responses to the stress of call duty for first- and second-year pediatric house officers were assessed using reliable and well-validated measures administered to 34 house officers who were on call, compared with 27 house officers who were not on call. Negative mood increased from the pre- to post-24-hour period for house staff on call and decreased for house staff off call, yielding a significant interaction. State anxiety was significantly elevated for house officers who were on call. Perceived stress was comparable in the two groups. Hours of sleep obtained when on call were related to negative mood, anxiety and perceived stress, postcall. House officers' mood and anxiety levels were affected by the demands of call duty. Our findings are consistent with anecdotal reports of the effects of call and, to our knowledge, provide the first objective evidence of the effects of call on psychological state.