Thiazolidinedione (TZD) can stimulate insulin sensitivity by binding to transcription factors and indirectly acting on protein synthesis in mammary gland. In a previous sstudy using ewes in early lactation (70 d in milk, DIM) we observed a positive effect of TZD on milk protein content. This study evaluated the effect of TZD on protein synthesis in late lactating ewes. Twenty-three lactating ewes with a mean body weight of 60 ± 0.45 kg, producing 0.98 ± 0.20 kg milk/day, 120 ± 3 d in milk and fed with a TMR of corn silage plus concentrate were randomly assigned to 1 of 2 treatments in a complete randomized block design: 1) Control (iv. infusion of 5 mL/d of saline solution); 2) TZD (iv. infusion of 4 mg/kg of BW/day in 5 mL of saline solution). The experimental period lasted 15d (5 of adaptation and 10 of measurements). Milk samples were collected from all ewes on d 1, 3, 4, 6, 7, 10 and pooled by ewe to evaluate the concentrations of milk fat, protein, lactose, casein and total solids. Data were analyzed using the PROC MIXED of SAS with ewe as random effect, and treatment, block and their interaction as fixed effects. There was no effect of treatment, block and their interaction for milk yield or for the concentrations and yields of protein, fat, lactose, and total solids. However, TZD reduced milk casein concentration (Control = 5.65 vs. TZD = 5.17%, P = 0.02). These results indicate that TZD does not stimulate the milk protein content in lactating ewes in late lactation.