A novel procedure is proposed to (1) quantify the level of physiological coupling existing between arterial blood pressure and heart rate variability during spontaneous behavior and (2) identify the relative contribution of the arterial baroreflex in the production of this coupling. The procedure is based on the estimation of the cross-mutual information (CMI) between systolic blood pressure (SBP) and pulse interval (PI, the reciprocal of heart rate) beat-to-beat values. Use of this statistical function provides a quantification of both linear and nonlinear components of the coupling between variables. The procedure has been preliminarily applied to data collected in two spontaneously behaving cats before and seven days after the opening of the baroreflex loop. This allowed us to determine the baseline level of the SBP-PI coupling in intact conditions and the remaining fraction of SBP-PI coupling surviving deactivation of the baroreflex control function. We observed that in intact animals the cumulative physiological level of linear and nonlinear coupling between SBP and PI corresponded to 50% and 35%, respectively, of the theoretical maximal coupling. After removal of the baroreflex influence CMI values drastically dropped with respect to the above baseline values (-76% and -67%, respectively). Thus, use of CMI indicates that the arterial baroreflex is the major determinant of the SBP-PI link, accounting for about 2/3 of the total measured coupling existing between. these variables.