The influence of some s-triazine herbicides on acid phosphatase (acid Pase) and phosphodiesterase (PDase) from corn (Zea mays L. var. ‘Orfeo’) roots was investigated. Terbutryn [N-(1,1-dimethylethyl)-N′-ethyl-6-(methylthio)-1,3,5-triazine-2,4-diamine] stimulated both phosphatases, whereas prometryn [N,N′-bis(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine] stimulated only the PDase. Atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and desmetryn [N-methyl-N′-(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine] inhibited both phosphatases to varying degrees. Atraton [N-ethyl-6-methoxy-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], prometon [6-methoxy-N,N′-bis(1-methylethyl)-1,3,5-triazine-2,4-diamine], and simazine (6-chloro-N,N′-diethyl-1,3,5-triazine-2,4-diamine) inhibited acid Pase. No effect was exerted by ametryn [N-ethyl-N′-(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine]. The enzyme assays and the kinetic parameters demonstrated that the interferences observed were due to an action on the synthesis of one or both enzymes rather than on the enzyme reactions. The types of the N-alkyl and the C1-substituent groups in the structures of the s-triazines tested appear important in determining the degree of the interference.