The dimensions of the small intestinal diffusion barrier interposed between luminal nutrients and their membrane receptors were determined from kinetic analysis of substrate hydrolysis by integral surface membrane enzymes. The calculated equivalent thickness of the unstirred water layer was too large to be compatible with the known dimensions of rat intestine. The discrepancy could be reconciled by consideration of the mucous coat overlying the intestinal surface membrane. Integral surface membrane proteins could not be labeled by an iodine-125 probe unless the surface coat was first removed. The mucoprotein surface coat appears to constitute an important diffusion barrier for nutrients seeking their digestive and transport sites on the outer intestinal membrane.

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Experiments are reported concerning an evaluation of sympathetic influences on heart rate and cardiac contractility in normal young adult humans during a stressful reaction time task. During the preparatory interval only vagal influences on heart rate change could be found which were related to concomitant somatic activity. In expectation of the shock and for a more sustained period thereafter, sympathetic influences became manifested on both heart rate and contractility which were independent of concomitant somatic activity. In a follow‐up study, the relationship was evaluated between blood pressure, as measured directly from the radial artery, and both contractility and heart rate. Sympathetic influences on the heart were not found to be secondary to depressor effects, although appreciable phasic decreases in blood pressure were sometimes found to follow the onset of large increases in heart rate and contractility. The data suggest that sympathetic influences on the heart are normally very minimal but are evoked by intense stress when the organism attempts to cope with the stress.

An investigation of sympathetic influences on the heart was undertaken in dogs using classical aversive conditioning procedures. For this purpose, heart rate and cardiac contractile changes were measured, the latter using techniques assessing the rate of change or slope at which either the muscles of the left ventricle contract, or blood is accelerated in the ascending aorta, or the pulse pressure wave ascends from diastole to systole. Sympathetic influences were found to be more clearly manifest in contractile rather than heart rate changes as indicated by the greater attenuating influence of beta‐adrenergic blockade on anticipatory and unconditioned responses. Anticipatory sympathetic influences on the heart were commonly acute, becoming minimal upon repeated exposure to the aversive procedures. The rate of change or slope measures appear to detect sensitively extrinsic sympathetic influences on contractility. They are minimally influenced by intrinsic effects on contractility such as heart rate as well as non‐contractile events such as diastolic blood pressure. The slope of the ascending limb of the pulse pressure wave appears to offer a means of assessing contractility in human Ss.

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