In monitoring the weight of heroin user units that were seized and tested in the laboratory, the evaluation found that as a result of the undercover operation there was a significant decrease of approximately 30 percent in the weight of the heroin user units after the police operation. Prior to the operation, the average weight per unit was 1.1 g; 3 months after the operation ended, the average weight per unit had decreased to 0.8 g and remained there for at least 4 months before beginning to increase. Division of the seizures into weight groups per distribution units was based on data from the total heroin seizures sent to the laboratory in 2001. The undercover operation was conducted in Lod a city near Tel-Aviv considered to be the main drug distribution center in Israel. The undercover operation ended in May 2003. The success or failure of such an operation had been typically measured by the number of arrests made, the hierarchical level of the dealers arrested, the number of drug stations closed down, and the decrease in heroin seizures following the operation. 2 tables, 5 figures, and 6 references