Equivalents of primary prostaglandins A and B as well as E and F were determined in the amniotic fluid of 64 clinically intact women in early pregnancy. The prostaglandin levels of the A and B groups were continuously lowered, between the ninth and 32nd weeks of pregnancy, while prostaglandin E revealed gradual rise from 57 pg/ml, in the ninth and tenth weeks, to 100 pg/ml, between the 29th and 32nd weeks. Prostaglandin F levels behaved similarly, between the 16th and 21st as well as between the 29th and 32nd weeks, mean values being 60 pg/ml or 75 pg/ml, respectively. High prostaglandin F levels, as recorded between the ninth and twelfth weeks of pregnancy (244 pg/ml), were probably attributable to mode by which amniotic fluid has been collected. Collection in that phase of pregnancy was transcervically, following cervix dilation during abortion, which differed from transabdominal amniocentesis in more advanced pregnancy. The above findings are discussed with regard to their physiological and clinical relevance.