The microsomal O-deethylation of a novel coumarin analog, 7-ethoxy-4-trifluoromethylcoumarin (EFC), to a fluorescent product was characterized. Results indicate that this analog provides a rapid, convenient and highly sensitive means to assay cytochrome P-450-mediated metabolism. Like microsomal 7-ethoxycoumarin (7-EC) O-deethylation, EFC O-deethylation responded to both phenobarbital was greater than that seen with 7-EC (5- to 6-fold over control after 50 mg/kg/day for 4 days in Sprague-Dawley rats compared to approximately 2-fold for 7-EC). Since the reaction was monitored by direct fluorometry of the product, any departures from linearity under a particular set of reaction conditions (e.g. with highly induced samples) were immediately apparent. In the absence of an NADPH-regenerating system, background drift was very low (less than 0.01 fluorescent units), so the sensitivity of the assay was limited primarily by that of the fluorometer employed. This makes the assay particularly useful in situations where test material is limited, e.g. when measuring activity in cultured hepatocytes. Its simplicity, reproducibility, and response to a variety of inducing agents also make it suitable for a rapid screening assay for cytochrome P-450 induction.