The distribution of sulphur in one ultrastructural type of neurosecretory cell (NSC) from the brain of the bug Rhodnius prolixus is investigated by X-ray microanalysis. In NSC in a storage phase, sulphur is concentrated in neurosecretory granules to about 17 times its level in adjacent neurons. However, the sulphur content of the cytoplasm of the NSC is considerably lower than that of adjacent neurons, so that the overall sulphur content of the NSC is only about four times that of adjacent neurons. Under these conditions the sulphur metabolism of the NSC is apparently more heavily devoted to the secretory function of the cell than comparison with other neurons would suggest. The high sulphur content of granules is attributed primarily to the presence of sulphur-rich protein. It is suggested that this protein may be analogous to vertebrate neurophysin. Analysis of the quantity and distribution of sulphur-containing compounds within NSC by X-ray microanalysis is suggested as the basis of a method for estimating relative rates of synthesis of neurosecretory products.