Treatment of adult rats for 28 days with 0.50% of simmondsin induced a reduction of food intake of about 40%. The degree of emaciation was the same in simmondsin treated rats (SM) and pair-fed controls (PF). Diurnal variation of plasma thyroid hormone concentrations was determined. The reduction in food intake induced a decrease of mean 24 h plasma T3, T4 and TSH concentrations. Plasma T4 concentrations were decreased more in SM than in PF at almost all times of the day. No difference was seen between SM and PF for plasma T3 and TSH concentrations. Liver outer ring deiodinating activity (type I) remained unchanged compared to controls in PF and SM, but liver inner ring deiodinating activity (type III) was increased 4 fold in both SM and PF. It was concluded that the food intake reduction induced by simmondsin produced a decrease in plasma TSH concentrations which in turn reduced plasma T4 concentrations. Decreased plasma T4 concentrations together with an increased degradation of plasma T3 resulted in lower plasma T3 concentrations.