The effects of baroreceptor reflex on mean cardiac (MCSNA) and renal sympathetic nerve activity (MRNA) were analyzed before and after atropinized (0.1 mg/kg, i.v.) states in conscious cats at rest. Resting values of MCSNA, MRNA, mean aortic pressure (MAP), and heart rate (HR) were 85 .+-. 6 imps/s, 76 .+-. 11 imps/s, 100 .+-. 4 mmHg, and 164 .+-. 10 beats/min, respectively. Both MCSNA and MRNA changed almost inversely to changes in the absolute MAP in the range of 90-140 mmHg. Within this pressure range the gain of baroreceptor-sympathetic system to the heart and kidney was 2.31 and 1.84, respectively. MCSNA as well as MRNA was reduced to the noise level at the MAP of 142 and 150 mmHg, respectively. With atropine, MCSNA and MRNA were inhibited centrally whereas HR increased to 192 beats/min. The increase in MAP caused by norepinephrine (2.1 .mu.g/kg, i.v.) was enhanced to 75 .+-. 7 mmHg by atropine from 31 .+-. 4 mmHg in control. The piecewise linear MAP-MCSNA and MAP-MRNA relationships changes to a remarkable clockwise hysteresis loop. During the rising MAP period, the gain of the baroreceptor-sympathetic system decreased to 0.91 and 0.97 in MCSNA and MRNA, respectively. During the returning MAP period, a delayed activation in MCSNA and MRNA occurred centrally. We concluded that the baroceptor reflex effect on MCSNA is larger than those on MRNA, and that the baroceptor control of MCSNA and MRNA is modified centrally by atropine in the awake cat at rest.