The aim of this study was to determine the effects of morphine on the biomass and development rate of Chrysomya albiceps (Diptera: Calliphoridae). C. albiceps, a well-known forensically important species which is among the first wave of faunal succession on human cadavers, which makes it a valuable source of information for the estimation of postmortem interval (PMI). Antemortem exposure to substances such as drugs and toxins may have an effect on the biomass and/or on the development rate of insects that feed on carcass, which may directly affect PMI estimation. In this study, three rabbits were administered 12.5, 25 or 50 mg/ml of morphine sulfate via ear perfusion over a period of 3 hours, and a fourth rabbit, which did not receive morphine, was used as a control. The rabbits were sacrificed using chloroform 30 minutes after morphine administration. The tissues were analyzed for the presence of morphine using HPLC-UV. Morphine was detected in all tissues of rabbits that received morphine, except in the bile and spleen of the rabbit which received 12.5 mg/ml dose of morphine. The presence of morphine in rabbit tissues retarded larval development rate, but accelerated the puparial development rate. The rate of development of C. albiceps larvae that fed on rabbits which received 25 and 50 mg/ml dosages of morphine was 9 days each. However, the rate of larval development was similar in the 12.5 mg/ml morphine group and the control; 6 days. Results of this study show that an underestimation of the postmortem interval of 72 h based on larval development and an overestimation of 24 to 48 h based on puparial development is possible if the presence of morphine in tissues is not considered. Moreover, the decreased larval development rate caused an increase larval length and weight compared with the control group. In this study, we found a strong correlation between the concentration of morphine administered and concentrations in rabbit tissues. In the estimation of PMI, it is recommended that effects of drugs such as morphine on the development of carcass colonizers be considered.