High- and low-pitched tones (CS+ and CS−) signalled baroreceptor stimulation or inhibition (US+ and US-) on 6-s conditioning trials (n= 128). Baroreceptor stimulation was induced by the phase-related external suction (PRES) method of Rau et al. (1992) in which a brief pulse of negative external pressure is applied to the neck at systole and one of positive pressure at diastole within each cardiac cycle (the reverse sequence is used for baroreceptor inhibition). Changes in heart period (R-R intervals) confirmed that PRES manipulated the baroreceptors in the presence of CS+ and CS− without habituation over conditioning trials. However, conditioned heart period responses were not observed on test trials (n= 32) in which CS+ and CS− were presented with the baroreceptor manipulation removed. Subjects were unable to state which CS had signalled baroreceptor stimulation and inhibition when given PRES-alone trials after the conditioning phase (differential attention thus controlled). These results (a) confirm that the differential effect of the two PRES stimuli was specific to the baroreceptors and (b) support earlier studies that have found that differential conditioning is impaired when CS−US relations are not processed in attention. We discuss implications regarding when baroreceptor firing might be discriminable and reinforcing.