The present study reports the kinetic inhibitory profile of 1-[3,4-dihydroxy-5-nitrophenyl]-2-phenyl-ethanone (BIA 3-202), a novel inhibitor of soluble catechol-O-methyltransferase (COMT) in rat liver. After an oral single dose (30 mg kg(-1)), there was a time-dependent recovery of enzyme activity from 98+/-2% inhibition at 30 min to total recovery at 24 h after treatment. The inhibitory effect produced by BIA 3-202 on soluble COMT was reversible after gel filtration of the samples. BIA 3-202 acted as a fast inhibitor of rat liver soluble COMT, interacting immediately with the enzyme after mixing. No differences were observed in the metanephrine formation rates (in nmol mg protein(-1) min(-1)) obtained without and with a 60-min preincubation with 30 nM of BIA 3-202 (1.30+/-0.02 and 1.35+/-0.03, respectively). The tight-binding nature of the inhibition produced by BIA 3-202 was evaluated by performing an Ackermann-Potter plot. The true K(i) for BIA 3-202, derived from the nonlinear regression analysis, was 0.19+/-0.02 nM. In substrate competition studies, an increase in the concentration of adrenaline resulted in a linear increase in IC(50) values for BIA 3-202. In conclusion, BIA 3-202 behaves as a reversible, potent and fast tight-binding COMT inhibitor that acts competitively at the substrate binding site of rat liver soluble COMT.