Loss of central 5-hydroxytryptamine (5-HT) reduces sympathetic tone to the vasculature in males. Typical responses of mean arterial pressure (MAP) and heart rate (HR) to phentolamine for male wild-type (WT) and TPH2−/− pups (A) and female WT and TPH2−/− pups (B). Note that phentolamine injection (at vertical dashed line) induced a smaller decrease in MAP in the male TPH2−/− pup vs. its WT counterpart, an effect not seen in females. C, top: average pretreatment values for the MAP of WT [open, nonhatched bars, n = 11 males (left) and 6 females (right)] and TPH2−/− pups [solid, nonhatched bars; n = 8 males (left) and 7 females (right)] as well as values, postphentolamine treatment, at 5, 50, and 500 μg/kg, indicated by the open, hatched (WT) and closed, hatched (TPH2−/−) bars. ^Significant effect of dose (3FRMA, P < 0.001); ††significant interaction between genotype, sex, and dose on the fall in MAP (3FRMA, P = 0.04). Bottom: same data as in top, expressed as change in MAP from pre- to post-phentolamine treatment, in WT (open circles) and TPH2−/− pups (closed circles).