The antitumor drug cisplatin causes neurological side-effects in patients treated with this drug. Since acetylcholine plays a key role in human neurotransmission we characterized the inhibitory effect of cisplatin on the enzyme, acetylcholinesterase. Enzyme activity was monitored spectrophotometrically using Ellman's method. The time for 50% inhibition (t1/2) was inversely proportional to the concentration of the cisplatin. The reaction was therefore assessed to have a bimolecular rate constant of 36.5 (mM min)-1. The Km and Vmax were both decreased by 45 and 48%, respectively by 7.0 mM cisplatin during the reversible phase while the Km was increased 138% and Vmax was decreased up to 65% in the irreversible phase. The nature of the inhibition was uncompetitive and complex irreversible at the reversible and irreversible stages respectively. The inhibition constants for reversible and irreversible steps were estimated as 1.12 mM and 97.70 (mM min)-1 respectively. The dissociation constant for the irreversible complex was 2.62 mM. These studies show that cisplatin is an uncompetitive inhibitor of acetylcholinesterase. Such effects may contribute, at least in part, to the neurotoxic effects associated with the use of cisplatin.