The renal artery of anesthetized dogs was perfused by a peristaltic pump with arterial blood. The pump rate could be modulated sinusoidally between 0.75 Hz and 0.0005 Hz. The frequency response of the renal arterial pressure to sinusoidal arterial flow of varying frequency and constant amplitude indicated the existence of a very slow autoregulatory pressure controlling mechanism, besides the autoregulation of flow. This fact is supported by the pressure responses to step flow increase, which show a very slow secondary pressure decrease. The time constant of this new autoregulatory phenomenon is in the order of 1000 sec. A simple linear second order model allows to simulate the renal arterial pressure flow relation (input impedance) as well as the characteristic step responses. Both the autoregulation of flow and the very slow autoregulation of pressure appear to be first order mechanisms.