Alphaxalone was a clinically used steroid anesthetic. Its analog delta 16-alphaxalone is nonanesthetic. The only difference between the two is the presence of a double bond at the hydrophobic end of the delta 16-alphaxalone molecule. This study determined the anesthetic potency of alphaxalone and delta 16-alphaxalone in goldfish and compared it with their effects on dipalmitoylphosphatidylcholine (DPPC) membranes and an alpha-helix polypeptide, poly(L-lysine). The goldfish EC50 values were: alphaxalone 5 mumol/L and delta 16-alphaxalone 80 mumol/L. Because these steroids are insoluble to water, the bulk of the steroid in water is absorbed by the fish. Larger containers hold more steroids than smaller containers at the same steroid concentrations. Then, EC50 values vary according to the size of the container. By assuming that the total amount of steroids in the container is distributed into the fish, the EC50 values expressed by the concentration in the fish body become 1.9 mmol/L for alphaxalone, and 30.5 mmol/L for delta 16-alphaxalone. A monoamino acid peptide, poly(L-lysine), can be formed into random-coil, alpha-helix, or beta-sheet. Addition of 0.07 mmol/L alphaxalone to the alpha-helix poly(L-lysine) partially transformed it to a beta-sheet structure. An equivalent change was observed with 3.0 mmol/L delta 16-alphaxalone. These values translate into 3.5 mmol/L for alphaxalone and 0.15 mol/L for delta 16-alphaxalone, when expressed by the concentration in the peptide. The change from alpha-helix to beta-sheet is accompanied by dehydration of the surface of poly(L-lysine). The steroids decreased the phase-transition temperature of DPPC membrane.(ABSTRACT TRUNCATED AT 250 WORDS)