Two techniques have been investigated to determine more accurately the quantitative riboflavin needs of the rat during reproduction. One procedure, that of determining the deposition of riboflavin in the developing fetal and placental tissues at several intervals during pregnancy, has revealed a very rapid increase in the requirement for this factor during the last four days of pregnancy. Prior to this time, and in fact for the majority of the reproductive cycle, the full-grown rat appeared to have little need for riboflavin above that of the non-pregnant adult;The development of the placental tissue influenced the total need for riboflavin to a negligible extent;Metabolism studies conducted on pregnant and non-pregnant stock females maintained on a ration providing approximately 100 mcg. of riboflavin per day indicated that the diet provided a large surplus during the early portion of pregnancy and supplied some excess oven during the final days of gestation, as judged by the excretion of the vitamin by the kidneys. Females receiving this ration produced large litters of active young with satisfactory birth weights. The young were well stocked with riboflavin and there was no depletion of the maternal tissues during pregnancy;It was observed that stock females 92 days old were still accumulating body stores of riboflavin; those animals have a higher dietary need for riboflavin during reproduction than do older animals;Thiamine and riboflavin assays of the developing fetal tissues and those of the female rat during pregnancy indicated that the need for riboflavin was lose than that for thiamine. This quantitative relationship between the two vitamins Is particularly interesting in view of the present recommended allowances for the two vitamins during pregnancy in women;In the formulation of rations satisfactory for reproduction in rats, it appears that a daily intake of 100 mcg. of riboflavin is desirable during the last third of pregnancy when the demand for this vitamin reaches a maximum. This intake provides a considerable surplus during earlier phase of the pregnancy period;Thiamine requirements exceed those for riboflavin during pregnancy in the rat, although this relationship does not follow during maintenance of the adult female. The date from present study suggest that a daily intake of 125 mcg. thiamine is ample for reproduction during the period of maximum need. This amount supplies a large excess of the vitamin in the first half of the gestation period.