Our demonstrations of the widespread and valuable properties of fluoroimidazoles as biological substrates and inhibitors have provided the impetus for the synthesis and evaluation of other substituted imidazoles in biological systems. A host of previously unknown analogues of histamine and histidine have now been obtained with substituents at C-2, C-4, or both. Novel synthetic methods have been developed to provide analogues containing halogen, amino, formyl, cyano, nitro, carboalkoxy, and trifluoromethyl groups. The last compounds were found to be particularly labile above pH 8 and are being investigated as receptor affinity labels for use in vivo. Toward this end, the isomeric trifluoromethyl-His analogs of TRF have been synthesized and are being evaluated for prolactin release and for irreversible binding to TRF receptors. A linear free energy correlation has been achieved (by computer analysis) for predicting the electronic effects of other substituents on the reactivity of the trifluoromethyl group in various imidazoles. A unique method has been developed for the synthesis of 2-hydroxyl-aminoimidazoles, compounds which are expected to be selectively cytotoxic for cancer cells.