Tens of millions of Americans are subject to drug and alcohol tests each year. How did this biochemical surveillance become so routine and normalized? This article examines the historical emergence, contestation, and gradual acceptance of the first biochemical surveillance technology, the drunkometer—the predecessor of the breathalyzer—which analyzed blood alcohol content in the breath. In the 1930s, public concerns over drunk driving grew alongside mistrust of rapidly expanding policing and security apparatuses. In this context, the drunkometer emerged as a seemingly commonsense solution to the problems of drunk driving and police mistrust, using a scientific technology to decenter law enforcement testimony and, in turn, streamline difficult cases. By examining contestations to the drunkometer in newspapers, court records, and other public documents, we detail how courts and the public gradually accepted the biochemical constitution of the body as the legitimate domain of authoritative institutions. We introduce the concept of “chemical citizenship” to analyze this authoritative institutional use of biochemical technologies to extract and deploy “the truth” about substance use as a mode of governing access to the rights and benefits of citizenship. We apply this concept to demonstrate how the drunkometer laid the foundation for contemporary drug testing.