Simultaneous inhibition of the RTN and ROb decreased the systemic CO₂ response by 51%, an effect greater than inhibition of RTN (-24%) or ROb (0%) alone, suggesting that ROb modulates chemoreception by interaction with the RTN (19). We investigate this interaction further by simultaneous dialysis of artificial cerebrospinal fluid equilibrated with 25% CO₂ in two probes located in, or adjacent to, the RTN and ROb in conscious adult male rats. Ventilation is measured in a whole body plethysmograph at 30 degrees C. There are four groups (N=5): 1) probes correctly placed in both RTN and ROb (RTN - ROb); 2) one probe correctly placed in RTN, one incorrectly placed in areas adjacent to ROb (RTN - peri-ROb); 3) one correctly placed in ROb, one probe incorrectly placed in areas adjacent to RTN (peri-RTN - ROb); 4) neither probe correctly placed (peri-RTN-peri-ROb). Focal simultaneous acidification of RTN-ROb significantly increased V_E up to 22% compared to baseline, with significant increases in both f and V_T. Focal acidification of RTN-peri-ROb increased significantly by up to 15% compared to baseline. Focal acidification of ROb and peri-RTN had no significant effect. The simultaneous acidification of regions just outside the RTN and ROb actually decreased _E by up to 11%. These results support a modulatory role for the ROb with respect to central chemoreception at the RTN.