The crustacean pigment-translocating hormones, the red pigment-concentrating hormone (RPCH), an octapeptide, and the light-adapting distal retinal pigment hormone (DRPH), an octadecapeptide, are the first invertebrate neurohormones to be fully characterized. Studies with both purified and synthetic hormones show that, in certain decapods, RPCH is a general pigment-concentrating hormone (PCH), affecting the pigments of all kinds of chromatophores (erythrophores, xanthophores, leucophores and melanophores); the DRPH seems to serve not only light-adapting function, but also act as a general chromatophore pigment-dispersing hormone (PDH). The two hormones thus function as antagonists when regulating the color-adaptation of the decapod crustaceans. PCH activity is widely distributed within the arthropod endocrine systems. The first characterized insect neurohormones, the locust adipokinetic hormones (AKH I and AKH II), show close structural similarities to the crustacean hormone, indicating a common evolution of some of the arthropod neurohormones. Physiological studies of the three hormones (RPCH, AKH I, and AKH II) and their synthetic analogs show that they crossreact, i.e., they all exhibit pigment-concentrating activity when tested on decapod crustaceans, adipokinetic activity when tested on locusts, and hyperglycemic activity when tested on cockroaches, although each of the hormones is more potent in its own system. Structure-function studies show, however, that quite different binding-site requirements exist for the hormones in activating their receptors on the various target tissues. The physiological specificity in their action therefore seems to depend on a differential evolution of the hormone receptors.