Two families derived from a maize somaclone previously found to tolerate the exposure to 2.4 microM (0.2 kg a.i. in 400 1 ha~i of water) glyphosate were evaluated genetically for herbicide tolerance. The lines were self-crossed and crossed with three inbred genotypes showing significant variation in tolerance to the herbicide. Seedlings of the families this way obtained were evaluated in a growth chamber following a treatment with 2.4 microM glyphosate at the three-leaf stage. Visual injuiy rating, dry weight, shoot height and carbon-exchange rate were scored two weeks after the application of the herbicide. General combining ability effects were significant, suggesting that additive gene action is important in conferring tolerance to glyphosate. The results strengthen the possibility that additional factors, not related to the properties of the main target of herbicide action, the activity of the shikimate pathway enzyme 5-eno/-pyruvyl-shikimate-3-phosphate synthase, may provide the basis for increased tolerance to glyphosate.