1. While the gender bias associated with coronary artery disease has been suggested to be partially accounted for by the protective effects of oestrogens, the role of testosterone remains unclear. The aim of the present study was to determine whether vasorelaxation could be affected by acute administration of testosterone with and without 17 beta-oestradiol. 2. Precontracted porcine coronary artery rings were relaxed with sodium nitroprusside (SNP), levcromakalim, bradykinin (BK) or A23187. At 1 nmol/L, testosterone impaired relaxations to BK and A23187, while the same concentration of 17 beta-oestradiol potentiated levcromakalim- and SNP-induced relaxations. The impairment of relaxation responses by testosterone was reduced in the presence of 17 beta-oestradiol, while the enhancement by 17 beta-oestradiol was decreased by testosterone. 3. We demonstrate that a low level of testosterone can impair agonist-induced relaxation, an effect that is reduced by 17 beta-oestradiol. This further supports evidence indicating a detrimental role for testosterone in coronary artery disease and suggests that circulating levels of testosterone may undermine the beneficial effects of oestrogen in women.