An experiment with eight beef steers was conducted to determine the patterns of change for plasma concentrations of thyroxine (T4), triiodothyronine (T3), cholesterol and total lipid following the addition or withdrawal of the organophosphate, ronnel, from the diet. Steers were fed 0, 2, 4 and 8 mg ronnel/kg body weight daily in a Latin square arrangement of treatments with five 28-d periods. Plasma samples were obtained at -2, 1, 3, 5, 7, 9, 16 and 23 d of each period. Steers had not received ronnel before period 1 and, for periods 2 to 5, steers previously fed the lower levels (0 and 2 mg ronnel) were switched to the higher levels (4 and 8 mg ronnel) and steers previously fed the higher levels were switched to the lower levels. Treatment assignments for periods 1 and 5 were the same. Feed intake was similar for all treatments (7.8 kg/d) and daily gain was greater when ronnel was fed (.65 vs .88 kg). Plasma T4 concentrations increased positively in response to ronnel level within the first day after ronnel feeding began and reached a plateau within 3 d. Plasma T4 concentrations decreased following the withdrawal of ronnel, but the response was slower; a lower level being established within about 7 d. Changes in plasma T3 concentrations were small, but were concurrent with plasma T4 changes and were inversely related to ronnel level. Plasma cholesterol concentrations tended to be positively related to ronnel level, but changed slowly with time; plasma total lipid concentrations were not consistently affected by ronnel level. The results of this experiment demonstrate that ronnel has an immediate effect on thyroid function in beef steers.