Summary The rate of adenylate kinase activity from the C 4 plant Saccharum ravenae at saturating substrate and Mg 2+ concentrations is 7 times higher in the direction of ADP formation (AMP + ATP →2ADP) than in the reverse direction. At low Mg 2+ the reverse reaction is faster than the forward. This is due to a strong positive cooperation with Mg 2+ in the forward reaction, whereas the reverse reaction follows a normal hyperbolic rate curve. Kinetic properties of the enzyme are the same at either 7.2 or 7.8 pH values. Positive cooperation with Mg 2+ is also observed in adenylate kinase extracted from Zea mays, A triplex halimus and Cyperus rotundus (C 4 plants) as well as from Kalanchoe tubiflora and Agave americana (CAM plants of the malic enzyme decarboxylation pathway). It is suggested that the enzyme might be regulated through the diurnal changes in stromal Mg 2+ concentrations.