The effects of the herbicide glyphosate (K + salt) on isolated maize mitochondria have been investigated. Protein synthesis, oxygen uptake (state 3 and state 4 respiration) and passive swelling were inhibited at concentrations in the 10~6-10~2 M range. No decrease of the respiratory control ratio {RCR) or stimulation of ATPase activity by glyphosate (K + salt) were observed. It is concluded that the previously reported decrease of the RCR and ATPase stimulation by glyphosate (isopropylamine salt) were probably due to the isopropylamine moiety or to impurities of the technical product.