WhenMusca domesticafemales were maintained on sugar, most did not develop their ovarian follicles past stage 4. However, a 4 h pulse feeding of protein diet to these flies stimulated ovarian maturation and increased haemolymph ecdysteroid and vitellogenin levels. Ecdysteroid levels were significantly higher than in the the sugar-fed flies within 8 h of protein feeding, reaching a peak of 36 pg/ul at 28 h, and then decreased to starting levels by 52 h. The higher ecdysteroid concentrations were found in samples from houseflies that had stage 5 and 6 ovaries. Decreasing levels of ecdysteroid were associated with flies that had ovarian stages of 7 through 9. Diet influenced the concentration of specific ecdysteroids in the haemolymph. Pulse feeding protein increased the concentration of RIA-positive material that co-migrated on HPLC with ecdysone by 13 fold, but had no effect on the concentration of the material that co-migrated with 20-hydroxyecdysone. The concentrations of polar materials and RIA-positive materials co-migrating with 20, 26-dihydroxyecdysone and makisterone A, also increased after protein feeding. Vitellogenin levels were significantly higher than the controls by 12 h after protein feeding, reached a peak of 16 ug/ul at 20 h and then decreased to control levels by 52 h. Significant ovarian development was noticed by 8 h after protein feeding and required 114 h for completion.