The kinetic analysis of the recombination of NO with some ferric hemoproteins was performed by the use of flash photolysis and stopped-flow methods. The rate constants for recombination of NO with ferrimyoglobin obtained by the two methods were identical with each other in the whole pH range. The rate constants decreased with an increase in pH, giving a pK value of 8.5 (cf. 5.2 x 10(4) M-1 s-1 at pH 6 and 1.3 x 10(4) M-1 s-1 at pH 10). The kinetic difference spectra of NO-ferrimyoglobin at 1 ms after flash were identical with the difference spectra of NO-ferrimyoglobin minus ferrimyoglobin at corresponding pHs. Unlike NO-ferrimyoglobin, NO-ferrihorseradish peroxidase gave different kinetics of NO binding for the two methods. Between pH 9.4 and 11.8, the velocity of NO recombination with the enzyme measured by flash photolysis remained constant, but that by the flow method decreased with increasing pH. Below pH 9.4, both methods gave an identical value of 1.9 x 10(5) M-1 s-1. The kinetic difference spectra showed that the acid form, but not the alkaline form, appeared first upon photolysis of NO-ferrihorseradish peroxidase even at alkaline pH. The acid form of peroxidase isoenzyme C re-formed the NO complex, while that of peroxidase isoenzyme A produced a mixture of the NO complex and the alkaline form. The data obtained here were compatible with the assumption that the formation of the alkaline form of the enzymes is the coordination of OH- at the sixth position, which is vacant at acidic pHs.