In the USA and Canada, methylcyclopentadienyl manganese (MMT) is currently added to gasoline to replace tetraethyl lead as an antiknock fuel additive. Manganese concentrations in roadside soil and plants are increasing and correlated with distance from the roadway, traffic volume, plant type, and microhabitat. Radish (Raphanus sativus L.) seedlings were treated for either five or thirty-five days with different levels of manganous chloride (0-1000 ppm). Metabolic heat rates (q) and respiration rates (R-CO2), measured calorimetrically, indicated severe stress at Mn concentrations between 10 and 100 ppm and at temperatures above 20 degrees C. Predicted growth rates (R-SG) also decreased in these circumstances.