The relationship between the PQ-segment slope on ST-segment depression during vigorous exercise was examined in 26 apparently healthy females between 18 and 26 years of age. Each subject performed 2 submaximal cycle ergometer exercise tolerance tests (trial A and trial B) on nonconsecutive days wherein the following variables, as delta scores, were measured; P-wave amplitude (microvolts), PQ-segment slope (uV!sec), and J-point at 0 and 60 msec (uV). Each variable was measured by both visual and computer averaging. The degree of reproducibility within and between trials differed for the visual and computer averaged measures. Generally higher reproducibility was found with computer averaging particularly within trial B (r =0.63-0.89, p<O.OI). Trial b served as a basis for assessment of PQ-segment slope effect on ST segment response. Computer analysis of frequency distribution for responses revealed a greater frequency of downsloping PQ-segment with clinically significant ST-segment depression (>50 uV) at both 0 and 60 msec after the J-point in lead II. However, there was a greater percentage (91%) of flat PQ-segment slopes with clinically significant ST-segment depression at J-point 0 msec in lead V5. These findings suggest possible influence of lead selection on the measurements of the PQ-segment slope and ST-segment. Implication of clinical application would be to use lead VS for diagnosing CHD and by measuring ST-segment depression at J-point 60 msec. However when screening exercise ECG tests in apparently healthy women use J-point at 0 msec.