Fetal hemoglobin (HbF) and adult hemoglobin (HbA) synthesis was studied in fetal baboons, Papio cynocephalus, to determine the normal pattern of hemoglobin production during fetal development. Fetuses ranging from 53 to 180 days gestation (term gestation 184 days) were used. Erythroid cells were incubated with 3H-L-leucine, and the rates of globin chain synthesis and the distribution of radioactivity into hemoglobin intermediates and completed hemoglobin molecules were determined. Gamma chain synthesis accounted for approximately 97% of the total nonalpha chain synthesis up to 140 days gestation; beta chain synthesis accounted for the remainder. After 140 days gestation, approximately equal quantities of gamma and beta chain were synthesized in the bone marrow. Prior to 140 days gestation, total alpha chain synthesis was 30% greater than total non-alpha chain synthesis, while there was balanced chain synthesis after 140 days gestation. During the period of excess alpha chain synthesis, fetal erythrocytes contained a large pool of alpha-hemoglobin (alpha chain with heme attached) molecules uncombined with beta or gamma chains. In view of the possibility that alpha chains may have a lower affinity for gamma chains than beta chains, excess alpha chain synthesis may be required to maintain low levels of free gamma chains.