The heart nerves of Praunus flexuosus Leach ( = Macromysis flexuosa White) have been stained with methylene blue applied in various ways. As a rule the animals were injected with a solution prepared immediately before use by mixing 1 part of 0–5 % solution of methylene blue in distilled water or of rongalit white with two parts of sea water. The injection was made into one of the posterior segments of the abdomen with the needle pointing forwards. When three or four animals are treated at the same time, usually in at least one of them the staining reaction takes a satisfactory course and its progress can be observed under the microscope, owing to the transparency of the tissues. In a few minutes after the injection some of the nerves are already visible. Later the coloration becomes more general and more intensive, but soon begins to fade out and can disappear completely.