Human milk contains many different components which interact locally in the gastrointestinal tract and stimulate some systemic functions. Among these substances are protaglandins. We obtained samples of foremilk and hindmilk in 8 lactating women by manual expression before and after 9 a.m. and 9 p.m. on the 3rd day post partum. The following values were found for prostaglandin E₂: foremilk, 162 ± 135 pg/ml; hindmilk, 198 ± 148 pg/ml. The values for 6-ketoprostaglandin F_1α were: foremilk, 818 ± 386 pg/ml, and hindmilk, 795 ± 487 pg/ml. The values for thromboxane B₂ were: foremilk, 3,557 ± 2,770 pg/ml and hindmilk, 4,476 ± 4,053 pg/ml. The results showed a great variability without statistical differences between paired values of the four groups. This dispersion may be produced by the procedure for extraction and by individual basal levels.