An olive tree was treated twice in the field with C-14-dimethoate (237.7 mu Ci, 2.4 g) and C-14 residues were determined in the olive fruits at harvest. The fruits were crushed and pressed to extract the crude oil, then refined by neutralization, bleaching and deodorization. The crude oil contained 14.1% of the total C-14 in the olive fruits. Neutralization resulted in a reduction of C-14 by about 50% of the total C-14 residues in oil. Bleaching and deodorization processes further reduced the C-14 residues and the refined oil contained 31.6% (which corresponds to 4.4% of C-14 residues of the total C-14 in olive fruits) of the total C-14 in the crude oil. Industrially extracted crude oil was fortified with C-14-dimethoate at 1.8 mg kg(-1) (0.02 mu Ci) level and subjected to the same refining process. A sharp decrease in the amount of C-14 was observed by neutralization and the amount of C-14 remaining in the refined oil was about 7.3% of the total C-14 in the crude fortified oil. The data suggest that the C-14 residues in the aged and the fortified oil amples were not of the same nature. The terminal C-14 residue in the refined oil obtained from the field experiment did not contain dimethoate and/or its oxon.