Abstract: A diverse set of 57 heterocyclic organic chemicals, consisting of a five-membered unsaturated ring of four carbon atoms and one oxygen (furans), or sulfur (thiophenes), or nitrogen (pyrroles) were evaluated for reactivity with thiol and acute aquatic toxicity assays using glutathione (GSH) as a model nucleophile and the ciliate Tetrahymena pyriformis, respectively. ...