Conventionally, immunization protocol require an initial priming immunization followed by an antigen boost for adequate immunity and long-term immunization. In this study we were evaluate the immunocastration efficacy and longevity of immunization with a single-dose GnRH vaccine (Repro-Bloc (TM)) in young male rats. Rats in the vaccine group (VAC, n: 35) were treated with GnRH vaccine as single-dose (200 mu l) and the control group (C, n: 15) were also injected with saline solution. Blood samples from all rats were taken monthly for 6 months and at 12th month. GnRH antibody titers, testosterone and leptin levels were determined by commercial RIA kits. Rats at 6th and 12th months were sacrificed for histologic analysis of testes. GnRH antibody levels were found similar in the VAC and C groups (P>0.05) at -1 week. As compared to the C, GnRH antibody levels (pg/ml) of the VAC group at 1st, 2nd, 3rd and 4th months rose continuously and reached the peak level at 5th month, decreased at 6th month and increased again by the 12th month (P<0.05), whereas in the C group GnRH antibodies weren't different (P>0.05). Compared to the unchanged testosterone levels within the process in C group, serum testosterone levels of the VAC group were lower at 2nd month and higher at 3th month, and thereafter lower (P<0.05). Leptin levels were no difference between groups (P>0.05). There were significant differences in the testicle weights, sperm motility, sperm numbers and abnormal sperm rates between VAC and C groups at 6th and 12th months (P< 0.05). Histological examination showed no significant difference in testes between groups at 6th and 12th months (P>0.05). In conclusion, although the vaccine reduced the germinative and endocrine testicular function from 6th months to 12th months with the single-dose GnRH vaccine, the single-dose vaccination did not provide adequate effect in the genital organs in male rats. Further studies are needed to determine an optimal single-dose, longevity of the vaccine and route of administration for complete suppression on the genital organs in different animal species.