The mechanisms by which male sex hormones modulate cardiovascular function are a subject of contemporary interest. Several lines of evidence indicate that androgens can exert acute vasorelaxing effects. On the other hand, chronic exposure to androgens has been shown to promote increases in blood pressure and compromise renal function. In the present issue of Clinical Science, Malkin and co-workers show that testosterone replacement impairs vascular reactivity in men with androgen deficiency. These studies may shed light on the functional and therapeutic significance of the diverging acute and chronic cardiovascular effects of androgens.