Blood from ivermectin-treated dogs was tested against adult mosquitoes. Mosquitoes were allowed to bloodfeed on mixed breed dogs 4 h after dogs were given oral dosages of ivermectin. In test 1, Anopheles quadrimaculatus and Aedes albopictus fed on dogs that had been given ivermectin dosages of 0, 6, 12, and 24 micrograms/kg. In test 2, Culex salinarius and Ae. albopictus fed on dogs that had been given dosages of 0, 15, 30, 60, and 120 micrograms/kg. In both tests, mosquitoes were observed for mortality at 1, 12, and 24 h postfeeding. Surviving mosquitoes were observed for oviposition and egg hatching. In the first test, there was a significant increase in mortality and a significant decrease in number of eggs/female and egg hatchability in An. quadrimaculatus but not in Ae. albopictus (P < 0.05). An LD50 of 9.9 micrograms/kg (95% FL 6.0-13.8) was determined for An. quadrimaculatus. In the 2nd test, there were no significant differences in any variable for Cx. salinarius or Ae. albopictus, except that eggs from Ae. albopictus had reduced hatching at all dosages.