The authors report 25 cases of patients, average age 67 years with severe coronary or valvular heart disease, with conduction disorders. The conduction disorder occurred alone in 8 cases and was associated with a disorder of excitability in 17 cases. It was either obvious, as in 14 cases, or latent, as in 11 cases, and precipitated by various forms of treatment, the disadvantage of which was the negative dromotrope effect. This treatment was prescribed for permanent resting angina (amiodarone and prenylamine), heart failure (digitoxin) or excitability disorder (beta-blockaders or procainamide). 11 patients had one or several fainting attacks. Permanent electro-systolic pacing with stimulation on demand, is necessary in all patients to palliate the consequences of treatment. In 11 cases out of 25, prior temporary pacing permitted the authors to assess the efficacy of treatment. The high post-operative mortality (40%) is not due to the apparatus but depends on the severity of the coronary heart disease or heart failure in these patients, In 60% of cases, the result of stimulation was excellent and was maintained permanently.