A simple and sensitive spectrophotometric method is described for the determination of some hydrazine and pyrazolone derivatives. The determination is based on the formation of charge-transfer complexes between tetracyanoethylene as a π-acceptor and the studied drugs as n-donors in acetonitrile solvent. The spectra, various experimental parameters and the stoicheiometry and stability of the reaction products were studied. The complexes formed were found to absorb at 395, 380, 410, 420 and 440 nm for the complexes formed with phenelzine sulphate, isonicotinic acid hydrazide, hydralazine hydrochloride, amidopyrine and antipyrine, respectively. Beer's law is obeyed in the concentration ranges 20–80, 1–10, 2–40, 10–60 and 30–80 µg ml–1, respectively, for the studied drugs. The proposed method is simple and can be applied to the determination of the studied drugs in their pharmaceutical dosage forms.