The aim of this study was to investigate the effect of 17β-estradiol on mammary tight junctions in cows in late lactation. The experiment included five non-pregnant cows around day 290 in lactation. The cows received injections of 17β-estradiol for six days. The effect of exogenous 17β-estradiol on milk yield, milk composition and lactose in plasma and lactose in urine was investigated before, during and after the treatment. Milk yield decreased after 17β-estradiol injections and lactose in plasma and urine increased, showing an effect on the integrity of the mammary tight junctions. However, there was a delay between hormone injections and the decrease in milk yield and opening of tight junctions, indicating that other factors are involved. A high correlation between lactose in urine and blood plasma was found. More than 30% of the total lactose production was lost in urine after 17β-estradiol treatment.
