In 12 normal subjects, use of ear densitography permitted measurement of systolic time intervals (STI) during uninterrupted treadmill exercise to over 90 percent maximal heart rate on a modified Bruce protocol. Results for control (sitting leads to standing) leads to end-exercise at 18 minutes leads to 10 minutes' sitting recovery were--heart rate (HR): (77 leads to 861 leads to 171 leads to 98 beats per minute; preejection period (PEP): (106 leads to 111) leads to 49 leads to 110 msec; ejection time index (ETI) (351 leads to 330) leads to 380 leads to 366 msec; PEP/LVET (0.42 leads to 0.49) leads to 0.29 leads to 0.45. Heart rate increased steadily to 18 minutes. In contrast, the PEP/LVET decrease was almost complete by three minutes and both ETI and PEP responses were bimodal with respective plateaus between three and six minutes and three and nine minutes. All STI and HR curves showed rapid restitution during early recovery. The changes during treadmill exercise were numerically comparable to those during bicycle exercise at a common heart rate. The results indicate that it is technically feasible to measure systolic time intervals with precision during uninterrupted treadmill exercise; responses to treadmill exercise were in the direction expected and quantitatively comparable to bicycle exercise at comparable stress level. These obervations provide the technical and physiologic bases for applying this method in clinical treadmill exercise testing.