To determine if resting blood pressure (X1), highest attainable blood pressure (X2), and lowest attainable blood pressure (X3) affect magnitude of pressure change (Y) elicited by carotid occlusion (CO) and by certain drugs, responses to CO and intravenous injections of isoproterenol, histamine, acetylcholine, epinephrine, and norepinephrine were measured in 100 dogs. Drug dosages were 0.05, 0.5, 1.0, 5.0, and 10.0 [mu]g/kg. Twenty animals received each dosage of each drug. Analysis indicated Y was directly related to X1 and, inversely related to X3 (other variables fixed) for higher dosages of isoproterenol and histamine. For the higher dosages of epinephrine and norepinephrine, Y related directly to X2. An inverse X1-toY relationship was seen for the highest dosages of epinephrine and norepinephrine. It was concluded that the "law of initial value" was operating for the dosages mentioned, and that the physical limitation of X2 or X3 might explain the law. Response to CO related directly to X1, the average threshold resting pressure required for elicitation of the reflex being 60 mm Hg.