The aim of this study was to examine the progestin content and biosynthetic potential of the corpus luteum of the African elephant (Loxodonta africana). Luteal tissue was collected from nonpregnant and early, mid- and late pregnant elephants (n = 2 per group) shot in the Kruger National Park. Pieces of individual corpora lutea (2-3 per animal; 23 in total) were stored directly in ethanol before hormone analysis. Matching tissue pieces were incubated for 2 h with [3H]pregnenolone (2 x 10(5) c.p.m.), after which tissue plus medium were also stored in ethanol. Progesterone and 17 alpha-hydroxyprogesterone immunoreactivity in tissue extracts were determined by enzymeimmunoassay and radioimmunoassay, respectively, before and after reverse phase HPLC. Progesterone immunoreactivity predominated over that of 17 alpha-hydroxyprogesterone in all corpora lutea examined but concentrations of both hormones were very low (73-374 ng g-1 and 3-93 ng g-1, respectively after HPLC). There were no obvious differences in hormone concentrations in corpora lutea from animals at different reproductive stages. Progesterone and 17 alpha-hydroxyprogesterone immunoreactivity assayed before HPLC was 10-30 times higher than that measured after chromatographic separation. HPLC consistently revealed two large immunoreactive peaks associated with relatively nonpolar compounds, which together accounted for most (at least 75%) of all progesterone immunoreactivity measured. Large amounts of radioactivity with the same retention times as these peaks were also detected after HPLC in samples incubated with [3H]pregnenolone. Analysis of conversion products from four corpus luteum incubations indicated that between 52% and 84% of [3H]pregnenolone had been converted; 19-33% was accounted for by progesterone, and 12-50% by the two substances represented by the unidentified peaks. Subsequent GCMS analysis identified the two immunoreactive peaks as 5 alpha-pregnane-3 alpha-ol-20-one and 5 alpha-pregnane-3,20-dione (5 alpha-dihydroprogesterone). These results indicate that the major progestins contained within and biosynthesized by corpora lutea of African elephants are 5 alpha-reduced metabolites, and that progesterone and 17 alpha-hydroxyprogesterone are quantitatively of minor importance.