The reactions of glyoxylic acid with peripheral stores of noradrenaline and 5-hydroxytryptamine to provide a fluorescence histochemical method for their localization have been investigated. Incubation in glyoxylic acid, followed by drying and heating of whole mount preparations gives an intense and well localized reaction. For incubation, a concentration of 2% glyoxylic acid, buffered to pH 7 at room temperature for 30 minutes gives ideal results. The method is equally good if the pH is varied in the range 6 to 9 or if the tissue is stored in the incubation mixture for up to 6 hours. Ideal development of the fluorophore requires an initial excess of moisture in the tissue, that this moisture is driven off during development, and that the tissue is protected from further moistening. A suitable method of achieving these ends is to heat partially dried tissue at 100°C for 4 minutes and then cover it with paraffin oil. 5-hydroxytryptamine can be readily distinguished from noradrenaline because it forms a fluorophore after reaction at pH 3.5, whereas noradrenaline does not. Both amines can be visualized after incubation at neutral pH. Comparison with the formaldehyde vapour technique reveals three main advantages (and no disadvantages) of the glyoxylic acid method: (1) it gives a finer localization with higher fluorescence yield, (2) the glyoxylic acid method is less susceptible to variations in procedure and, (3) it is both simpler and quicker to apply.