Microcirculatory observations and measurements were made by in vivo microscopy in 35 germfree and 26 conventional rats. The rate of vasomotor activity, "vasomotion," of precapillary arterioles was found markedly decreased in the germfree animals.

All precapillary vessels in the germfree rats were markedly hyporeactive to the catecholamines, epinephrine and norepinephrine, as compared to similar vessels of conventional rats. The vessels in the germfree animals were also less responsive to vasopressin but not angiotensin.

A greater lumen:wall ratio of primary arterioles, but not of secondary arterioles and metarterioles, found in germfree animals is related to change in vessel lumen alone without concomitant change in wall thickness.

The germfree rat is characterized by possessing a hypotonic, catecholamine-refractory mesoappendix microvasculature.

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