10Carotid sinus (SIN), aortic (AO) and subclavian (SUB) baroreceptor activities were recorded from intact nerves during experiments using cardiopulmonary bypass in 11 rabbits. These activities (number of impulses/second) were expressed as a percentage of basal activity observed before artificial circulation (AC). The preparation remained in a relative, "stable period" for up to 45 minutes. Then, a progressive decrease occurred in maximal arterial pressure (AP max) in the 11 experiments (fig. 2,4). 20 According to the level of arterial pressure the "stable period", our experiments can be divided into two groups: a) Group I (6 rabbits) with AP max greater than or equal 90 mm Hg; in this group, the correlation coefficients between pressure levels and nervous activities calculated during the period 0-70 min after the onset of AC, were positive (fig. 3) and statistically significant, as we could expect (AO, p less than 0.02; SIN, p less than 0.01; SUB, p less than 0.01). b) Group II (5 rabbits) with AP max less than 90 mm Hg; surprisingly, in this group no positive correlation was found between levels and nervous activities, which were generally very high (fig. 5). 30 Experiments were performed in 18 other rabbits in order to check the nature of the higher activities recorded in group II. These high activities were not of chemoreceptor or efferent origin, and, thus, were thought to be indeed really from baroreceptive fibres. 40 Ten experiments under AC were carried out to test the reactivity of baroreceptors in response to induced increase and decrease of pressure. The responses were qualitatively normal (i.e. an increase of pressure produced an increase in the nervous discharge), but in group II, the operative point was set at a higher level (fig. 6). 50 A possible, time-dependent, shift in the baroreceptor response curve under AC can account for these findings by giving, in some cases the illusion of a negative correlation between baroreceptor activity and arterial pressure (fig. 8). 60 The above mentioned shift in the baroreceptor response curve could be related to a change in sympathetic control of baroreceptors,resulting in a higher activity, and initiating and/or maintaining hypotension.