An adapter for extending a sensing and stimulation delivery lead in a body stimulation system, such as a cardiac pacer or cardioverter defibrillator, includes a resilient, electrically insulative housing with a longitudinal barrel extended through the majority of the housing length for the insertion of a lead terminal connector. Two spaced apart transverse bores are formed in the housing, open to barrel and to the housing exterior. Two connector assemblies are mounted in the housing and positioned to receive respective pin and ring terminals of the lead when inserted into the barrel. Each connector assembly can be adjusted to secure the lead in the housing, by a tool inserted through one of the transverse bores. Each connector assembly is isolated from the housing exterior by a seal means mounted in the associated bore. Each seal means includes a seal member elastically deformable to admit a tool to the associated connector assembly via a passage through the seal member. A rigid retaining sleeve, secured within the associated bore, surrounds and slightly compresses its associated seal member to ensure that the passage remains closed when not admitting the tool.