Lepidium meyenii (Maca) is a Peruvian hypocotyl that grows exclusively between 4000 and 4500 m in the central Andes. Maca is traditionally employed in the Andean region for its supposed fertility-enhancing properties. The aim of this study was to test the hypothesis that Maca can prevent high altitude-induced testicular disturbances. Adult male rats were exposed for 21 days to an altitude of 4340 m and treated with vehicle or aqueous extract of Maca (666.6 mg/day). The lengths of the stages of the seminiferous epithelium and epididymal sperm counts were obtained at 0, 7, 14 and 21 days of exposure. The stages of the seminiferous tubules were assessed by transillumination. A dose-response study was also performed at sea level to determine the effect of Maca given to male rats at doses of 0, 6.6, 66.6 and 666.6 mg/day for 7 days on body weight, seminiferous tubule stages and epididymal sperm count. The length of stage VIII and the epididymal sperm count were increased in a dose-dependent manner in Maca-treated rats but treatment reduced the length of stage I. At the highest dose, sperm count increased 1.58 times, the length of stage VIII increased 2.4 times and the length of stage I was reduced 0.48 times compared with the value at dose 0. Exposure to high altitude resulted in a reduction in epididymal sperm count after 7 days and lower values were maintained up to 21 days. Altitude reduced spermiation (stage VIII) to half and the onset of spermatogenesis (stages IX-XI) to a quarter on days 7 and 14 but treatment with Maca (666.6 mg/day) prevented these changes. Data on transillumination and epididymal sperm count in the Maca-treated group exposed to high altitude were similar to those obtained at sea level. Maca increased the sperm count on day 21 of exposure to high altitude to values similar (1095.25 +/- 20.41x10(6) sperm, means +/- S.E.M.) to those obtained in the Maca-treated group at sea level (1132.30 +/- 172.95x10(6) sperm). Furthermore, in the Maca-treated group exposed for 21 days to high altitude, epididymal sperm count was higher than in the non-treated group at sea level (690.49 +/- 43.67x10(6) sperm). In conclusion, treatment of rats with Maca at high altitude prevented high altitude-induced spermatogenic disruption.