The regulation of blood pressure by the baroreceptor reflex was examined in anesthetized dogs. The receptor feedback path was broken by surgical intervention, the input (carotid sinus pressure) was controlled by an external servo system, and simultaneous records of input and output (systemic arterial pressure) were obtained. A variety of pressure waveforms were applied to the carotid sinus so that the nonlinear behavior could be explored thoroughly. Most of the input waveforms consisted of relatively small signals about an operating point in the normal blood pressure range. The nonlinear behavior of the reflex was illustrated in a number of ways. Square waves, sinusoids, and short pulses produced asymmetric arterial pressure waveforms. An overshoot in the arterial pressure always followed the rising edge of the input square wave, but was generally absent following the negative-going part of the square wave. Either positive or negative pulses caused a transient drop in arterial pressure. The sinusoidal response was often distorted, with the falling portion of the output waveform steeper than the rising portion. The system behaved as a rectifier, or envelope-detector, as illustrated by 1) the decrease in mean arterial pressure when either the amplitude or the frequency of a sinusoidal input was increased and 2) the appearance in the arterial pressure waveform of a component correlated with the envelope of a modulated carrier. Another nonlinear phenomenon was the decrease in low-frequency sinusoidal gain caused by the addition of a high-frequency sinusoid to the sinus pressure waveform. This behavior indicates that if closed loop behavior in the intact animal is to be predicted from the open loop sinewave response, the effects of the cardiac pressure pulse must be considered. A model was constructed from standard electronic components to simulate the input-output behavior of the baroreceptor reflex. The model was able to simulate the response to high- and low-frequency sinusoids and to reproduce the essential features of the square-wave and pulse responses. On this basis the model was considered adequate for simulating the performance of the baroreceptor reflex in certain open loop and closed loop situations.