The acute oral MLD of parathion to ewe and ram lambs of 20 kg body weight was approximately 40-50 mg a.i.jkg body weight. The insecticide was not markedly cumulative in its toxic effects.Pastures were topdressed with granules of 20% parathion at the rates of 3.4 kg and 6.7 kg a.i.zha and groups of four sheep were grazed on the pastures after withholding periods of 0, 7, 14, and 28 days. Sheep in the O-day groups developed slight scouring and their blood cholinesterase activity was severely depressed. None of these sheep died and their subsequent recovery was uneventful. In the other groups the only evidence of exposure to parathion was depression of blood cholinesterase activity, which was progressively less as the length of the withholding period increased. No parathion was detectable in the fat of the sheep that had the highest exposure to parathion on the treated pastures.Over a 3-month period parathion residues in topsoil (0-2.5 em) decreased from 6.5 ppm to 4.7 ppm and from 10.7 ppm to 8.5 ppm for application rates of 3.4 kg and 6.7 kg parathion/ha, There was little penetration of insecticide below the top 2.5 em of soil, and no parathion was found in the soil 8 months after treatment.Residues of parathion on pasture were more persistent in this grazing trial than in a subsequent ungrazcd plot trial. Parathion was not systemic in pasture, as residues were present apparently only on the plant surfaces.