The effect of pre- and postpartum selenium (Se) supplementation in ewes on thyroid hormone (T3) levels were evaluated. The relationship between plasma and milk Se concentration and serum T3 levels in ewes and their newborn lambs were analyzed. Twenty 90 d of gestation primiparous crossbreed hair ewes were selected and distributed randomly into two groups. The first group received subcutaneous Se supplementation (0.1mg of sodium selenite/Kg BW) at the 8th, 5th prepartum and 1st postpartum wk while the other was control without Se injection. Plasma and serum maternal samples were collected weekly since 8th prepartum week to delivery. At delivery and weekly until 8th postpartum wk, plasma, serum, colostrum and milk samples were taken from ewes. Plasma and serum samples were collected at 48 hours, 1st, 2nd, 3rd, 5th and 8th, postpartum wk from newborn lambs. Muscular and thyroid histological changes were studied. Results demonstrated significant positive relationship between maternal plasma Se and serum T3 in supplemented and control ewes (r = 0.69 to 0.72, P < 0.05). Increase in T3 concentration in supplemented ewes and their lambs until the 8th postpartum week was demonstrated (P < 0.001).There was positive relationship between milk Se concentration and serum T3 in the newborn lambs of the supplemented group (r= 0.84, P < 0.01), while the relationship was negative in the control group (r = -0.89, P < 0.01). Muscular and thyroid pathological changes were described and were independents of Se-treatment suggesting a pre- experimental Se deficiency period.