Abstract 1. 1. Changes in plasma concentrations of thyroxine (T 4) and triiodothyronine (T 3) were studied during spontanepus molt in 28-month-old white leghorn hens. The molt was accomplished over a long time period (2–5 months) with intermittent body and wing feather shedding. 2. 2. Plasma T 4 levels in molted hens did not rise in molting periods and were rather lower than those in non-molting hens, while plasma T3 levels in molted hens rose significantly during heavy molts. 3. 3. The results suggest that in natural molt of hens, the increased thyroidal activity is not necessarily a prerequisite for molt except that more T 3 is needed for thermoregulation due to the loss of feather insulation.