In drunk-driving investigations, the natural dissipation of alcohol in the bloodstream, alone, does not constitute a per se exigent circumstance that justifies taking a blood sample without a warrant or consent. McNeely was stopped by a police officer for speeding and crossing the centerline. After declining to take a breath test to measure his blood alcohol, he was taken to a hospital where a blood sample, revealing blood alcohol levels above the legal limit, was taken. The officer did not obtain a search warrant prior to the taking of the sample and McNeely did not agree to it. The trial court suppressed the results of the blood test, ruling that the search violated McNeely’s Fourth Amendment rights, and the Missouri Supreme Court affirmed. Held: Absent exigent circumstances, search warrants are required before a compelled blood sample may be taken for use as evidence in a criminal investigation. The U.S. Supreme Court rejected the state’s position that the natural dissipation of alcohol constitutes an exigent circumstance justifying a per se rule allowing the taking of a sample without a warrant or consent. Rather, the court directed that the matter be determined on a case-by-case assessment of the totality of the circumstances, in which the dissipation element is a factor in evaluating whether an exigency exists.