Two types of heart block which occur extensively in families in the Republic of South Africa are reported. A type I heart block tends to have the pattern of a right bundle-branch block and/or left anterior hemiblock occurring individually or together, and manifesting clinically when complete heart block supervenes, either with syncopal episodes, Stokes-Adams seizures or sudden death. The condition is inherited as an autosomal dominant gene and appears to be progressive in nature; the risk to life appears to be greatest at 3 particular periods:at or soon after birth, during puberty and the early 20s, and again towards middle age. The type II condition also appears to be progressive and is inherited as an autosomal dominant gene. The pattern, however, tends to develop along the lines of a sinus bradycardia with a left posterior hemiblock, again presenting clinically as syncopal episodes. Stokes-Adams seizures or sudden death when complete heart block supervenes. Both conditions are likely to be widely prevalent throughout the Republic of South Africa. The pathogenesis is discussed in relation to the patterns of the conduction disturbances.