Mitochondria were isolated from green leaves and stems of the glycophyte Pisum sativum and the halophyte Suaeda maritima. The preparations oxidized malate, succinate, and 2-oxoglutarate as well as externally added NADH. Acceptor control ratios were about 2.8 for mitochondria from Pisum and 1.8 for mitochondria from Suaeda oxidizing malate+pyruvate in 125 mM sodium chloride. The mitochondrial fraction was contaminated with chloroplast fragments which resulted in relatively low rates of oxygen uptake when these were expressed on a protein basis. The addition of sodium chloride at concentrations greater than 200 mM considerably reduced the rates of oxygen uptake by both species in the presence and absence of phosphate acceptor (ADP). Acceptor control ratios were reduced and there was a marked decline in the ADP/O ratio. Sucrose at equivalent molar concentrations had a much less drastic effect on the mitochondria. There was no significant difference in the effects of these two solutes on mitochondria from the two species and the similarity of response is discussed in relation to the cytoplasmic ion content of the halophyte.