A device and method for the rapid on-site detection of inhibitors of enzymes, such as acetylcholinesterase, is described wherein the device contains all reagents added to a sample pad containing dried releasable enzyme creating a reaction mixture wherein inhibitor deactivates the enzyme, while said reaction mixture travels via a longitudinal membrane to a distal porous pad containing a substrate for the enzyme. The reaction of the enzyme and the substrate results in a product that can generate a measurable signal such as color, fluorescence or luminescence to serve as a reporter. Signal that is generated at this reaction zone is inversely proportional to inhibitor concentration in the test sample. A device containing two such strips, one for a test sample, the other for a negative control fluid as an onboard comparator is described. A purpose-built reader or an illuminating device, such as, containing an incandescent light source, a diode, a UV light source or any other illumination source that is suitable for the reporter or mere visualization is used to determine the level of reporter.