UNLABELLED OBJECTIVE; To observe the plasma concentration change of endothelin-1 (ET-1) and the histomorphological changes in penile cavernous body after administrating a continuous low dose of tadalafil into spontaneously hypertensive rats (SHR). METHODS Forty SHR rats were randomly divided into four groups of blank group, placebo group, continuous low-dose group and intermittent dose group with 10 rats in each. The animals were administrated with different doses of tadalafil via an intragastric route. To observe the plasma concentration change of ET-1 and cellular morphological changes of penile cavernous tissue in SHR after four weeks, the immunohistochemical method of SABC was employed to detect the expression of alpha-smooth muscle actin (alpha-SMA) in penile corpora cavernous of different groups. RESULTS The plasma levels of ET-1 were lower in continuous low dose group (12.0 +/- 1.6) and intermittent dose group (14.3 +/- 1.7) as compared with the blank (18.0 +/- 1.9) and placebo groups (17.6 +/- 2.2) (all P < 0.01). And there was significant difference between the continuous low dose and intermittent dose groups (P < 0.05). The A-value level of absorbance in alpha-SMA were significantly different with the blank (0.53 +/- 0.03) and placebo groups (0.52 +/- 0.05) (P > 0.05) and it was lower in the continuous low dose group (0.29 +/- 0.03) than the intermittent dose group (0.38 +/- 0.03) (P < 0.05). It can be observed microscopically that in blank group and placebo group: endothelial cell and vascular smooth muscle cells of penile cavernous tissue in SHR had a disorderly distribution. The disrupted continuity and integrity of endothelial cell were also found. And they improved in continuous low dose group and intermittent dose group, especially the former; it can be viewed under TEM that the endothelial cells of penile cavernous tissue in SHR had mostly reverted to normal condition in the continuous low dose group and its ultra-structure indicated that the tissue morphology improved. CONCLUSION A continuous low dose of tadalafil can improve the function and structure of vascular endothelium in penile cavernous body. Thus it provides rationales for a continuous low dose of PDE5 inhibitor in the treatment of erectile dysfunction.