The specificity of rat liver biliverdin reductase was examined with the help of a series of synthetic biliverdins. The mixture of the four biliverdin isomers obtained by the chemical oxidation of protohemin I, protohemin XI, protohemin XIV and harderohemin were used as substrates of biliverdin reductase and were compared with the mixture of biliverdins IX alpha-delta. Biliverdin reductase (molecular form 1) from rat liver efficiently reduced the isomer mixtures of biliverdins I, XI, XIV and harderobiliverdins to the bilirubins in the presence of NADPH. The enzymatic reduction of the different biliverdin types was studied in the presence of different NADPH analogues. NADPH could be replaced by NADH, 3-acetyl NADPH and deamino-NADPH with retention of a good substrate activity only in the case of biliverdins of types I and IX and harderobiliverdins. Biliverdins XI and XIV were efficiently reduced only in the presence of NADPH and an excess of NADH. Bactobilin III-alpha was also very efficiently reduced by biliverdin reductase in the presence of both NADPH and NADH but not in the presence of the other analogues. These results indicate that biliverdin reductase reduced bilitriene acids substituted with non-polar and polar residues.