This study monitored fecal cortisol and corticosterone levels in 14 black rhinoceroses (Diceros bicornis) and in seven white rhinoceroses (Certotherium simum) under various conditions of captivity, including translocation. Free cortisol and free corticosterone were measured in methylene chloride extracts of feces, using high-performance liquid chromatography (HPLC). The extraction-assay method was validated for quantitative measurement of these hormones by mass spectroscopy analysis, chemical derivitization, and radiolabel tracking and recovery. Both cortisol and corticosterone were extractable from feces and routinely detectable by HPLC. In three nonstressed, captivity-adapted white rhinoceroses monitored across 21 days of routine activity, fecal cortisol ranged from 2.0 to 7.3 ng/g dry feces and corticosterone from 4.0 to 10.8 ng/g dry feces, with no observable trend. Matched plasma, urine, and fecal samples in these rhinoceroses yielded corticosterone:cortisol ratios of 2.0:1.0, 2.7:1.0, and 2.2:1.0, respectively. Both black rhinoceroses (n = 5) and white rhinoceroses (n = 4) exhibited higher fecal cortisol (6.9- to 10.0-fold) and corticosterone (3.2- to 4.5-fold) levels in association with restraint-translocation than in limited free-roaming conditions. In five black rhinoceroses monitored across 6 wk after release from translocation, fecal levels of both cortisol and corticosterone decreased significantly between week 1 and weeks 4-6. In general, cortisol and corticosterone paralleled each other, with cortisol exhibiting a greater range of response. Measurement of either hormone in feces appears to be reliable for adrenal axis monitoring in the white and the black rhinoceroses.