The effect of pathotoxin from Helminthosporium maydis on the growth, respiration, ion leakage and ATP content of intact roots was determined for susceptible (cms-T cytoplasm) and resistant (normal cytoplasm) corn seedlings. The results were correlated with the effect of the pathotoxin on energy-linked reactions of isolated mitochondria. H. maydis pathotoxin caused a rapid, but reversible, inhibition of root growth with effects observable after only 30 min of treatment. Tissue respiration, however, was not affected in a 2 h period and changes in the rate of phosphate uptake or release by treated roots did not reveal evidence of selective inhibition over a 3 h period. A decrease in the content of cellular ATP and an increase in the rate of ion leakage (as shown by conductivity measurements of washing solutions) were observed with susceptible seedlings only at high pathotoxin concentrations. The results with intact tissues do not correlate well with the effect of the pathotoxin on isolated mitochondria, since the isolated organelles are affected immediately and irreversibly. Phosphate uptake and energy-linked volume changes by isolated mitochondria were inhibited by the pathotoxin. It is concluded that mitochondria, in situ, do not appear to be the primary site of action of the pathotoxin.