OBJECTIVE To study the expressions of Erk1/2 and PKB/Akt in the corpus cavernosum of castrated rats and investigate their action mechanism in the development of erectile dysfunction (ED) after castration. METHODS We randomly divided 20 eight-week-old SD rats into Groups A (sham-operation) and B (castration), and, 4 weeks after the operation, determined the level of serum testosterone (T) and the expressions of P-Erk1/2 and P-PKB/Akt proteins (integrated optical density/area, IA/area) and those of Erk1/2 and PKB/Akt mRNA (Marker: GAPDH) in the corpus cavernosum of the rats by immunohistochemical staining and RT-PCR. RESULTS Four weeks after the operation, the serum T level was significantly decreased in Group B in comparison with A ([10.090 +/- 3. 026] nmol/L versus [1.339 +/- 0.642] nmol/L, P < 0.05). Erk1/2 and PKB/Akt expressed in the corpus cavernosum of both groups of rats. The expressions of Erk1 and Erk2 mRNA and P-Erk1/2 were significantly higher in Group B (0. 840 +/- 0.062, 0.876 +/- 0.141 and 0.142 +/- 0.020) than in A (0.479 +/- 0.090, 0.599 +/- 0.100 and 0.119 +/- 0.029) (P < 0.05). But no statistically significant differences were found in the expressions of PKB/Akt mRNA and P-PKB/Akt between Groups B (0.974 +/- 0.040 and 0.164 +/- 0.036) and A (0.942 +/- 0.054 and 0.162 +/- 0.025) (P < 0.05). CONCLUSIONS Erk1/2 and PKB/Akt expressed in the penile tissues of both castrated and sham-operation rats. The increased expression of P-Erk1/2 in the corpus cavernosum may be involved in the development of ED in castrated rats.