Four major toxic steroidal alkaloids are contained in skin extracts of the Colombian arrow poison frog Phyllobates aurotaenia: (i) batrachotoxinin A (LD50 1 mg/kg mice), C24H35NO5, 3a,9a-epoxy-l4b,18/3-(epoxyethano-N-methylimino)-5/3-pregna-7,16-diene-3/3,1 la,20a-triol, the structure of which was elucidated by X-ray crystallography of the 20a-p-bromobenzoate; (ii) pseudobatrachotoxin, an extremely labile alkaloid of unknown composition which on standing spontaneously forms batrachotoxinin A; (iii) batrachotoxin, C31H42N2O6, the most toxic principle (LDSo 2 pg/kg mice), which has now been recognized as the 20a ester of batrachotoxinin A with 2,4-dimethylpyrrole-3-carboxylic acid, and (iv) homobatrachotoxin, the former "isobatrachotoxin," C32H44N2O6 (LDjo 3 pg/kg mice), now formulated as the 20a ester of batrachotoxinin A with 2-ethyl-4-methylpyrrole-3-carboxylic acid. A partial synthesis of batrachotoxin was achieved by the reaction of the anhydride of ethyl chloroformate and 2,4 dimethylpyrrole-3-carboxylic acid with batrachotoxinin A. The analogous esterification with the anhydride of the fully substituted 2,4,5-trimethylpyrrole-3-carboxylic acid gave a homolog of batrachotoxin which was more stable and twice as active (LD50 1 pgfig). The 20a ester of batrachotoxinin A with 1,2,4,5-tetramethylpyrrole-3-carboxylic acid was much less active than batrachotoxinin A itself. Reductive opening of the 3,9-hemiketal oxygen bridge of batrachotoxin with sodium borohydride leads to an acid-sensitive dihydrobatrachotoxin with 1/250 of the activity of batrachotoxin.