OBJECTIVE: In traditional medicine, the seeds of Thai Mucuna pruriens (T-MP) are used to treat male dysuria and are believed to enhance fertility. However, information pertaining to the toxicity of T-MP and its interaction with other properties is limited. This study was thus conducted to evaluate the antioxidant capacity and subacute toxicity of T-MP in the reproductive system. METHODS: Total phenolic content and antioxidant capacity of T-MP seed extract were determined using total phenolic content, 2,2-diphenyl-1-picrylhydrazyl and ferric reducing antioxidant power assays. Male and female adult rats were treated orally with T-MP at a dosage of 150 or 300 mg/kg body weight for 14 consecutive days. Sex hormones and functional parameters in the liver and kidney were evaluated. Histopathology of all tissue was conducted using Masson's trichrome staining. Sperm parameters, including concentration, morphology, acrosome reaction status and DNA damage, were also examined. Expression of tyrosine phosphorylated protein (TyrPho), androgen receptor and A-kinase-anchoring protein 4 (AKAP4) were investigated using the Western blot technique. RESULTS: T-MP seed extract contained phenolic compounds and exhibited high antioxidant capacity with no toxicity at the tested doses. It did not affect liver or kidney function parameters in the male rats, but increased estradiol, aspartate aminotransferase and alanine aminotransferase levels in the females. Additionally, it decreased serum progesterone and alkaline phosphatase levels in female rats. Serum and intratesticular testosterone levels were significantly lower in male rats that received a high dosage of T-MP. Histopathological changes were not observed in any tissue treated with T-MP. T-MP also significantly increased sperm concentration (but did not affect sperm parameters), and enhanced testicular TyrPho protein and androgen receptor and expression of AKAP4 in sperms. CONCLUSION T-MP seed extract exhibited antioxidant capacity and was not harmful to reproductive tissues. It also had a phytoestrogenic effect on females and increased the expression of testicular and sperm markers of male fertility.