The enthalpy change for anabolism is needed to model the growth/respiration relation in plants. If all CO2 production is assigned to catabolism, the anabolic reaction becomes C-substrate --> C-products + xO(2) with an enthalpy change, DeltaH(b). Four methods are proposed for determining DeltaH(b): (a) From the difference in the heats of combustion of substrate and anabolic products (i.e. newly grown tissue). (b) From the composition of newly grown tissue and application of Thornton's rule. (c) From independently measured values of the specific growth rate, R-SG, and of the product (R-SG DeltaH(b)). The product (R-SG DeltaH(b)) equals (-DeltaH(CO2) R-CO2 - R-q) where R-CO2 is the specific rate Of CO2 production by respiration, DeltaH(CO2) is the heat of combustion of respiratory substrate per mole Of CO2 and R-q is the specific metabolic heat rate. DeltaH(b) is then calculated as the ratio (R-SG, DeltaH(b))/R-SG. (d) From (DeltaH(b) = -(R-q/R-CO2 + DeltaH(CO2)) [(1 - epsilon)/epsilon] where epsilon is the substrate carbon conversion efficiency obtained from a total carbon balance. The first three methods have been tested and compared on oat seedlings and the last on corn seedlings. DeltaH(b) values from all four methods are in reasonable agreement despite the different assumptions involved.