Seasonal and gestational variations of triiodothyronine (T3) and thyroxine (T4) plasma concentrations were evaluated in 12 pregnant and 6 non pregnant adult Sahel Peulh ewes. Blood samples were drawn from all animals at different dates for radio-immunoassay. In this particular breed of sheep, plasma triiodothyronine and thyroxine levels did not present significant variations from the beginning of the cool season (December) until the end of the dry warm season (May). On the contrary, a highly significant rise of hormones (P less than 0.01) was observed at the onset of the humid warm season (June). Moreover, a significant decrease of thyroid hormone plasma levels (P less than or equal to 0.05) greater for thyroxine than triiodothyronine was observed at the end of gestation, particularly in twin pregnancies. At birth, the thyroxine plasma level was higher in single than in twin lambs (139.6 +/- 66.5 vs 110.5 +/- 43.8 ng/ml).