Two forms of farnesyl pyrophosphate synthetase were separated from hog liver extracts by DEAE-Sephadex chromatography. They were designated as farnesyl pyrophosphate synthetase A and B, in order of elution. Both enzymes catalyzed the exclusive formation of E,E-farnesyl pyrophosphate from isopentenyl pyrophosphate and either dimethylallyl pyrophosphate or geranyl pyrophosphate. They also showed no detectable differences in pH optima, molecular weights, and susceptibilities to metal ions. However, the catalytic activity of the synthetase B was greatly stimulated by the addition of common sulfhydryl reagents. This stimulation was the result of conversion of the synthetase B into the synthetase A. Conversely the synthetase A was converted into form B when it was dialyzed against a buffer solution containing cupric ions. It is suggested that the formation and cleavage of disulfide bond(s) is involved in the interconversion between the two forms.