Vector control is a serious concern in developing countries such as Egypt. The present study has been carried out to evaluate the larvicidal activities of 1,2,4-triazine derivatives against 3^rd larval instar of the mosquito “Culex pipiens" under laboratory conditions. To achieve this aim, the median lethal concentrations (LC₅₀) of the tested compounds and their effects on larval development and adult emergence were assessed. Adult females' fecundity and egg hatchability were estimated on the survivors of treatment. It was evident that 1,2,4-triazine compounds have a toxic effect on Cx. pipiens. The estimated LC₅₀ values of 1,2,4-triazine were 0.457, 0.333, and 2.047 mg/L for MSA 35, MSA 102, and the reference insecticide “pymetrozine", respectively. One week after treatment, both the compounds MSA 35 and MSA 102 were more effective (higher mortality rate and shorter lethal time) than the reference insecticide. The LC₅₀ of MSA 102 induced a significant prolongation in the development time and less adult emergence compared with the control. Moreover, adult females surviving the treatment of MSA 102 as larvae, laid no eggs, while those treated with MSA 35 showed reduced hatchability compared with the control. In conclusion, the larvicidal potency of the tested 1,2,4-triazine derivatives is suitable for application against Cx. pipiens mosquitoes and supposed to be a new alternative to other commercial insecticides like pymetrozine.