The function of glutamate dehydrogenase was studied in cultured sweetpotato (Ipomoea batatas) nodal explants. The glutamate debydrogenase was fractionated to charge isomers. Supplementation of the growth medium with either naphthaleneacetic acid or benzyladenine in the presence of 20 mM NH4NO3 induced normal growth (type 1 sweetpotato). The basic and acidic charge isomers of glutamate dehydrogenase were not suppressed. Combined supplementation with 70 mM NH4+ and either 1 mg · L-1 benzyladenine or 0.1 mg · L- naphthaleneacetic acid caused growth retardation (type 2 sweetpotato) and the suppression of the basic charge isomers. Combined supplementation with 45 mM NH4+ and either 1 mg · L-1 benzyladenine or 0.1 mg · L-1 naphthaleneacetic acid induced normal growth (type 3 sweetpotato), but the acidic charge isomers were suppressed. Combined supplementation with benzyladenine and naphthaleneacetic acid suppressed all the charge isomers and abolished amination by the enzyme, thereby causing severe growth retardation. The type 1 and type 3 sweetpotato glutamate dehydrogenases were more aminating (Michaelis constant K(m) = 12.5 and 14.8 mM NH4Cl, respectively) than type 2 sweetpotato glutamate dehydrogenase (K(m) = 82.6 mM NH4Cl). The differential growth retardations which accompanied the three phases of the suppression of the aminating charge isomers are evidence that the enzyme is aminating in vivo and that it employed that activity in the regulation of sweetpotato growth and differentiation.