High quality radioiodinated neuropeptides are essential to radioimmunoassays (RIA) and receptor binding assays. Approaches of direct and indirect labeling of neuropeptides with 125Iodine (125I) are compared. An HPLC equipped with an in-line gamma detector and UV absorbance detector was used to evaluate selected labeling methods and products. Treatment of [Y(1)]-adipokinetic hormone-I ([Y(1)]-AKH-I) with chloramine-T caused oxidative damage, whereas enzymatic labeling with lactoperoxidase in the presence of H(2)O(2) produced a good yield of intact, apparently monoiodinated peptide. Labeling of the FMRFamide-related peptide (YGGFMRFa), with chloramine-T apparently formed the methionine sulfoxide, which subsequently could be reduced with dithiothreitol. Products of high specific activity typically are achievable.