Plasma and fecal progestagen patterns of female (n = 10) vicunas (Vicuna vicuna ) were determined about 1 to 2 mo before and until 4 mo after breeding. The vicunas were caught wild and were penned at the Lauca National Park (Chile, 4470 m above sea level) for 7 mo (December to June). Plasma and fecal samples before and during the mating period (January to March) were collected 4 to 5 times weekly, and once or twice weekly thereafter. The samples were analyzed by enzymeimmunoassays (EIA) using antibodies against progesterone and 20alpha-dihydroprogesterone. High performance liquid chromatography (HPLC) separations confirmed that progesterone and 20alpha-dihydroprogesterone predominated in the plasma, whereas in the feces several unconjugated, immunoreactive progestagen metabolites containing either a 20-oxo- or a 20alpha-OH-group occurred. The coefficients of correlation (n = 409; P < 0.01) between matched plasma and fecal samples were 0.39 and 0.53 for 20-oxo- and 20alpha-progestagens, respectively. Elevated (5 to 6 d) plasma and corresponding fecal progestagens after mating indicated cyclic corpus luteum activity in 5 of the animals. After the mating period (23.2 +/- 3.3 d), corpus luteum function in these 5 animals persisted, as it did in 3 other animals that were not observed to be mating. The persisting corpus luteum function was demonstrated by increased mean plasma and fecal progestagen concentrations (> 1 ng/ml and > 100 ng/g, respectively). Mean plasma 20alpha-dihydroprogesterone concentrations exceeded that of progesterone by about 1 ng/ml (P < 0.01). The results demonstrated that in addition to plasma progesterone, plasma 20alpha-dihydroprogesterone and noninvasive fecal progestagen evaluations are useful, valid tools for determining corpus luteum function in vicunas.