For blood flow to be an effective agent for the control of heat exchange, it must occur in a region of the body where conduction resistance in the tissues is relatively high, and in an environment where external resistance to heat exchange is relatively low. If either of these conditions is not met, control of heat exchange by blood flow is not possible. Very small reptiles should not be able to control heat exchange by blood flow in any environment, unless they control blood flow specifically to appendages. Very large reptiles should be able to control heat exchange by blood flow only under certain conditions, such as in water, very high winds, or intense radiative heating. Otherwise, they should have little control. An optimum body size should exist for a reptile's ability to control heat exchange using blood flow. In air, this optimum body size for alligators appears to be about 5 kg. Theoretically, the optimum size should be substantially larger than 5 kg for reptiles heating and cooling in water.