Workload is an important determinant of human performance during complex dynamic tasks. We indirectly measured the mental workload of nine anesthesia residents during 19 cases using a secondary (subsidiary) task paradigm in which performance on an extra task (problems in mathematical addition occurring approximately every 45 s) was used as a probe of a subject's "spare capacity." Excess response time (above baseline) correlated with activities like "manual task" and "speaking with the attending anesthesiologist." The addition problem was either skipped or had a high excess response time (greater than 30 s) in 40% of presentations. The complexity of a case and the experience level of the resident interacted in determining the workload. Subjective workload ratings confirmed that induction of and emergence from anesthesia were periods of highest workload, with "attention" as the dominant cognitive requirement. The anesthesiologist's spare capacity may often be limited during anesthesia. These types of workload measurement techniques should be applied in controlled studies to better define the factors that increase workload.