We previously reported that exposure to low (22.5±0.2nm in diameter, 15.4±1.0μg/m3 in mass weight, 2.27×105/cm3 in mean number concentration), and medium (26.1±0.5nm, 36.4±1.2μg/m3, 5.11×105/cm3) concentrations of nanoparticle-rich diesel exhaust (NR-DE) for 1 and 2 months (5h/day, 5 days/week) significantly increased plasma testosterone in male Fischer 344 rats, whereas exposure to a high concentration (27.1±0.5nm, 168.8±2.7μg/m3, 1.36×106/cm3) did not. The present study attempts to clarify the mechanism of this elevation. Low and medium exposures to NR-DE for 1 and 2 months significantly increased steroidogenic acute regulatory protein (StAR)- and cytochrome P450 side-chain cleavage (P450scc)-mRNA and their protein expressions in the testis of rats, in which the elevation pattern was very similar to that of plasma testosterone levels. Interestingly, both exposure levels for 1 month significantly increased growth hormone (GH) receptor expression in the testis, and low exposure also increased testicular insulin-like growth factor I-mRNA levels and hepatic microsomal cytochrome P450 2C11-mRNA and their protein levels in rats. These two factors are thought to be related to growth hormone secretion. Disruption of testosterone biosynthesis by NR-DE exposure may be a mode of action for reproductive toxicity, which may, in part, be regulated by increasing StAR and P450scc expressions via GH signalling.