To test if 5HT2 receptor blockade affects ventilatory responses after VAH is already established, we measured the ventilation in the same rats (1) before and (2) after 7 days of CH, and (3) after 2 more days of CH with two acute doses of ketanserin (1 mg i.p./kg daily). This evidence concerns the gene HTR2A and cyclic hematopoiesis.