The cytochrome P450 (P450) superfamily metabolizes many endogenous signaling molecules and drugs. P450 enzymes are regulated by posttranslational mechanisms in vivo, which hinders their functional characterization by conventional genomic or proteomic methods. Here we describe a chemical proteomic strategy to profile P450 activities directly in living systems. Derivatization of a mechanism-based inhibitor with a "clickable" handle provided an activity-based probe that labels multiple P450s both in proteomic extracts and in vivo. This probe was used to record alterations in liver P450 activities triggered by chemical agents, including inducers of P450 expression and direct P450 inhibitors. The chemical proteomic strategy described herein thus offers a versatile method to monitor P450 activities and small-molecule interactions in any biological system and, through doing so, should facilitate the functional characterization of this large and diverse enzyme class.