The corpus luteum of African elephants produces high amounts of 5 alpha-reduced progesterone metabolites (5 alpha-pregnane-3,20-dione and 5-alpha-pregnane-3 alpha-ol-20-one), whereas progesterone itself is quantitatively less important, and plasma levels of progesterone during the estrous cycle in elephants are considerably lower than those of other mammals. The objective of this study was to compare the concentration of progesterone in plasma of Asian and African elephants as determined by specific progesterone assays with those of total immunoreactive progestagens containing a 20-oxo-group (20-oxo-P). These metabolites were determined by an enzyme immunoassay using an antibody against 5-alpha-pregnane-3 beta-ol-20-one, 3HS:BSA. Plasma of non-pregnant Asian (n = 4) and African (n = 4) elephants was collected at weekly intervals for periods of 8-15 months and at random intervals during pregnancy in one Asian elephant. High-performance liquid chromatography separation of plasma samples of both species demonstrated that in the 20-oxo-P assay, 5 alpha-pregnane-3,20-dione makes up similar to 60% of the total immunoreactive material. The progesterone and 20-oxo-P values during the estrous cycle showed a parallel pattern and were significantly correlated (P < 0.001; Asian: r = 0.80; y = 3.76 x -0.10; African: r = 0.75; y = 2.66 x -0.08). Progesterone and 20-oxo-P values in Asian and African elephants were <0.15 ng/ml during the follicular phase (weeks -4 to 0) of the estrous cycle; progesterone values during the luteal phase (weeks 2-9) were 0.60 +/- 0.03 and 0.53 +/- 0.03 ng/ml, and the 20-oxo-P values were 2.19 +/- 0.16 and 1.48 +/- 0.12 ng/ml, respectively. The 20-oxo-P values of the pregnant animal, although slightly higher, were comparable to those of non-pregnant elephants during the luteal phase. Total immunoreactive 20-oxo-P values are about three times higher than those of progesterone during the luteal phase, and 5 alpha-pregnane-3,20-dione is the major immunoreactive 20-oxo-P in the plasma of Asian and African elephants. (C) 1997 Wiley-Liss, Inc.