The electrophysiological effects of the cardiotropic drugs have been studied in man by the agency of endocavitary electrocardiography. The effects of drug combinations, which are often prescribed therapeutically, have been studied less often. The authors report the results of a preliminary study of the combination of deslanoside with ajmaline in 26 patients; its effects were compared with those using each drug separately. This combination seems to have true dromotropic effects; although deslanoside alone, in the doses used, does not modify conduction below the bundle of His, it can still act synergistically with ajmaline at this level. A detailed study of the pharmacological effects as a function of the original status of conduction shows that at the level above the bundle His, the dromotropic action is quantitively less on healthy conducting tissue than on abnormal tissue. The effects of ajmaline on the conduction times below the bundle seem to be similar whether or not there is any conduction defect under basal conditions. The difficulties in obtaining and interpreting such measurements in man are discussed in the hope of arriving at a general protocol for studying drug combinations.