The combination of levonorgestrel and quinestrol (EP-1) has been shown to have anti-fertility effects on several wild rodents, but the mechanism underlying these effects is poorly understood. We investigated the effects of EP-1 and each of its components, levonorgestrel (P) and quinestrol (E), on the fertility of Brandt's voles (Lasiopodomys brandtii) by using a gastric gavage method. The doses for EP-1, E and P were 1, 0.34 and 0.66 mg/kg body weight, respectively. Male voles (n = 98) were treated daily for 5 or 14 days, then the testes and epididymides were collected, weighed and examined histologically at 30 (D30), 60 (D60) or 90 (D90) days after the end of treatment. Four males were allowed to mate with normal females at D90. Female voles (n = 75) were treated for 3 days and a further 3 days after a 7-day interval. The uteri and ovaries were weighed and examined histologically at 15 (D15), 30 (D30) or 75 (D75) days after the end of treatment. Each of three females were mated with fertile males at D30 and D75, respectively. Our results indicated that quinestrol (E) significantly decreased the sperm numbers in the testes as well as the weight of the testes and epididymides, with both of these tissues showing obvious structural abnormalities, and significantly reduced the litter size and the pup weight for females mated with males of the E treatment group. For female voles, treatment with E, P or EP-1 resulted in no marked influence on the fertility status. These data indicate that quinestrol (E) alone has a significant anti-fertility effect on male Brandt's voles, but is ineffective in combination with levonorgestrel (P).