Neuro-depressing hormone (NDH) was separated from crude eyestalk extracts of Nephrops norvegicus and Carcinus maenas. The fractions containing NDH activity were tested for their effect on the spontaneous activity of motoneurons in isolated ganglia of Nephrops and on the locomotor activity of Carcinus. The same fractions extracted from either species were inhibitory on both preparations, thus indicating a cross-sensitivity to the hormone, and suggesting that the same substance is responsible for both effects. © 1979.