The serum pyridoxal 5-phosphate (vitamin B6) level was measured in 20 women with hyperemesis gravidarum before and after therapy with vitamin B6. Control specimens were obtained from 22 healthy pregnant, women during the first trimester, 18 healthy prgnant women during the last trimester and from 29 healthy non-pregnant women of comparable age. The determinations were performed by means of decarboxylation of L-tyrosine-1-14 C.