The 1994 Violent Crime Control and Law Enforcement Act created the Violent Offender Incarceration and Truth-in-Sentencing (VOI/TIS) Incentive project to provide funds for prison bed expansion, stipulating that up to 10 percent of the funds could be used for inmate drug testing. Beginning in 1999, the California Department of Corrections (CDC) conducted a pilot program at four CDC institutions that included random urinalysis drug testing and systematic drug interdiction practices. This article describes the CDC’s Drug Reduction Strategy (DRS) Project and provides an evaluation of its effectiveness, as measured by the rates of drug-positive inmate urinalysis tests and substance interdiction. The four sites were selected for similar institutional demographics and geographic diversity. Three of the sites were designated as intervention sites, while the fourth site was designated a comparison site. The DRS Project was implemented in two phases beginning in September 1999 and ending in January 2001. During phase I of the project, weekly random urine testing augmented existing urine testing procedures and was expected to have a systematic impact on inmates’ substance use. Phase II involved the systematic use of drug-detecting K-9 teams and drug-detecting equipment. Standard drug interdiction efforts continued at all sites during these two phases. A total of 3,559 inmates were selected for testing, with 203 (6 percent) refusing to provide a sample, and 173 (5 percent) being unavailable for testing. In the final sample, 3,183 inmates were tested, with 47 inmates (1.48 percent) testing positive for any substance. The most frequently detected drug at baseline was morphine, followed by THC, alcohol, phenobarbital, and barbiturates. Analysis of the data found that at baseline, the initial estimate of drug use was 8.85 percent. This rate was reduced to 3.74 percent at the completion of phase I. Further reduction was noted at the completion of phase II, with estimated drug use levels dropping to 2.07 percent. At the end of phase I, the three intervention sites had lower estimated substance use levels than the comparison site, which used traditional drug interdiction and detection measures. While the random urine testing continued during phase II and further reductions were found in the substance use levels at the four institutions, the addition of systematic drug interdiction efforts did not lead to greater reduction in substance use relative to urine testing alone. Policy implications are discussed. 5 tables, 1 figure, 2 notes, and 10 references