The fixed oil extracts from Thevetia peruvine, Datura stramonium and Acacia sp. were tested on Culex pipiens larvae. The estimated sublethal concentrations (LC50) were used in the present studies. The reproductive potential of females and the histochemistry of their ovaries were determined. The results indicated that oil treatments of larvae caused drastic changes in reproduction potential of female mosquitoes including the ovarian development in the first gonotrophic cycle, fecundity and fertility of treated females. Histological and histochemical studies revealed clear changes in the oocytes of females derived from treated larvae. Treatment with the oils reduced the vitellogenesis, synthesis of proteins, carbohydrates, lipid contents and DNA and RNA materials. The reproduction was always significant and may reflect the shortage in reproductive potential, the state of being disturbed. Generally, these fixed oils merit further investigations as a potential suppressant of Culex pipiens because their abilities to kill the early stages, inhibit growth and development of ovaries and reduce the reproductive potential of adult females. The three oils have variable activities but they are similar in their mode and site of actions.