Butyrylcholinesterase (BChE; E.C. 220.127.116.11.) was 260-fold purified from soluble fraction of rat intestine. The enzyme was composed of tetrameric globular form by nonreducing electrophoresis. Optimum pH value was determined as 7.2 after zero buffer extrapolation. Optimum temperature was examined as 37 C after zero time extrapolation. The enzyme showed marked substrate activation with positively charged, acyl-choline substrates. As a measure of catalytic efficiency, k(cat)/K-m values were determined as 16,210, 25,650, and 46,150 for acetylthiocholine (ATCh), propionylthiocholine (PTCh), and butyrylthiocholine (BTCh), respectively. When the catalytic efficiencies are compared, soluble isoform of rat intestinal BChE became increasingly efficient as the size of the acyl portion of the substrate increases; BTCh > PTCh > ATCh. Differently, the enzyme showed substrate inhibition with benzoylcholine (BzCh) and a k(cat)/K-m value of 21,190 was found. Triton X-100 inhibited more efficiently the rat intestinal BChE soluble isoform than it did the human serum BChE.