Rat fleas, Xenopsylla cheopis (Rothschild) and X. astia (Rothschild) were fed artificially on whole blood, milk and blood fractions in an attempt to identify the factor(s) which trigger ovarian maturation. Blood from the white rat, house rat, white mouse, frog, chick and man fed artificially induced vitellogenesis in over 50% of fleas of all combinations except in X. cheopis fed on chick blood. Gut distension alone had no influence on initiation of vitellogenesis as judged by feeding milk. Washed white rat blood cells resuspended in saline initiated vitellogenesis, but plasma did not. Addition to plasma of ATP, glutathione and serotonin did not initiate yolk deposition. However, raising the protein concentration of the rat plasma had a decided influence on vitellogenesis. The protein concentration of the diet appeared to be a decisive factor in initiating yolk deposition in these fleas. A change from whole blood to plasma diet caused all the mature oocytes to resorb in both the species of fleas.