CARLILL'S attempt1 to dispel confusion in the use of the Ohm's law analogy in blood circulation theory requires further clarification, since it is only the incorrect application of this analogy that needs to be abandoned not the analogy itself, but it raises an important point. This is confirmed by Burton2 when he insists that, for vascular circuits: Resistance to flow (R) = For a definition consistent with the electrical analogy we must have R = (P—p)/F where p is the critical closing pressure as referred to by Carlill. This gives F = P/R — p/R, which is of the form y = mx — c since p/R is constant for any single one of the lines in Carlill's Fig. 1 and gives the same result for dF/dP as in Burton's equation (4) derived from his incomplete expression of form y = mx.