In vitro kinetic studies of the reaction of hydralazine with acetaldehyde at physiological concentrations and pH were conducted. This reaction, which leads to the formation of 3-methyl-S-triazolo[3,4-a]phthalazine, may occur in the plasma and may represent an alternative pathway for hydralazine metabolism. The reaction of hydralazine with acetaldehyde followed second-order kinetics with an activation energy of 16.9 kcal/mole. At 37 degrees, the half-life of the reaction for a colution containing 2.3 microgram of acetaldehyde/ml and 1 microgram of hydralazine/ml was 4.5 hr. The rate increased with increasing acetaldehyde concentrations.