Histochemical, immunohistochemical, and biochemical evidence is reported showing that the ink gland of the cuttlefish Sepia officinalis contains a calcium-dependent isoform of nitric oxide synthase as well as an NMDA R1 receptor subunit localized for the most part in the immature inner cells of the epithelial layer of the gland. These results may be taken to implicate a hitherto unrecognized regulatory role of the glutamate-nitric oxide pathway in the maturation and metabolic activity of melanin-producing cells in the cephalopod defense system.