A recombinant expression vector containing Japanese eel (Anguilla japonica) testis cytochrome P450(11 beta) (11beta-hydroxylase) cDNA was introduced into COS-1 cells. Enzymatic activity of the expressed P450(11 beta) for corticosteroid synthesis was analysed by incubating transfected cells with 14C-labelled 11-deoxycorticosterone or 3H-labelled deoxycortisol as substrates. Thin layer chromatography of incubation medium revealed that a high percentage of 11-deoxycorticosterone was converted into corticosterone and 11-dehydrocorticosterone but no aldosterone was detected. Similarly, deoxycortisol was converted into cortisol and cortisone. These results show that eel P450(11beta) does not possess significant aldosterone synthesizing activity. Northern blot analysis detected a 1.8 kb transcript of P450(11beta) using RNA extracted from interrenals of untreated Japanese eel but no hybridization signal was apparent using RNA extracted from brain, spleen, heart, muscle or testis. Immunohistochemistry using an antiserum against P450(11beta) also revealed strong immunostaining in interrenal cells.