Fluorescence microscopy and chemical assay of catecholamines have been used to examine the changes in adrenaline content of the nerve fibres in the frog's heart after vagosympathectomy. There is an immediate increase in the content of the fine terminal fibres and it spreads up into the thicker fibres. It is followed by a loss of fluorescence and disintegration of the fibres. Their content is above normal for at least 48 h, and they have disappeared after 8 days. Isolated saline perfused hearts show a similar increase in adrenaline content, and this must be due to the synthesis of adrenaline within the peripheral parts of the neurons separated from their cell bodies. The continued synthesis is thought to reveal a normal 'depletion' of the nerve terminals due possibly to a failure of the re-uptake mechanism for released adrenaline, which may be being washed away by the very high blood flow past the terminals, before it can be taken up.