1. 1. In 38 normal men, there was a statistically highly significant inspiratory backward rotation of the mean QRS vector and forward rotation of the T vector as constructed from conventional leads with a mechanical vector analyzer, and opposite, though smaller, changes in maximal expiration. 2. 2. The downward movement of the mean QRS vector in maximal inspiration exceeded that of the T vector. 3. 3. The average respiratory changes of the vector magnitudes were statistically not significant, but in some subjects the changes were large. 4. 4. The angle between the spatial QRS and T vectors increased significantly in inspiration. 5. 5. There was a significant correlation between mid-respiratory position and respiratory changes in the elevation, but not in the azimuth of the vectors. 6. 6. Age was not found to affect the respiratory vectorial changes. 7. 7. The respiratory changes of mean QRS and T vectors, constructed from the conventional leads and from the electrically orthogonal X, Y, Z leads of the SVEC III system, in 22 men, were in the same direction but different in extent. The interindividual variability was large in both lead systems. 8. 8. In 4 men, the respiratory changes of spatial QRS and T loops were oscillographically recorded with 7 different lead systems (SVEC IIa, SVEC III, Briller, Duchosal, Frank, Grishman, and Wilson-Burch). 9. 9. Discrepancies between respiratory changes of QRS and T vectors were noted in all lead systems investigated. Changes of spatial loop configurations were also seen in all systems. 10. 10. The differences of vector orientation and magnitude between the several lead systems, both in regard to the initial mid-respiratory position and to the respiratory changes, in each of the subjects, were tremendous, the extremes exceeding the biologic variability. One lead system (Wilson-Burch) showed a change of the QRS vector opposite in direction to that in all other lead systems. 11. 11. In 5 subjects, the spatial vectors were studied over the whole range of vital capacity in fractionated respiration. It was proved that variations in vital capacity do not account for the large interindividual differences that were found. 12. 12. In 4 patients with ventricular bigeminy, the vectorial respiratory changes of the normal regular and the premature ventricular beats were different. 13. 13. The experimental results cannot be explained adequately on the basis of anatomic respiratory changes alone.