A NEED has been felt for a precise terminology to distinguish four types of animal hormones by their actions1,2. (1) ‘Metabolic’ hormones stimulate or inhibit clearly defined metabolic processes. This term is in general use for such hormones as thyroxine and a secretion from the insect corpus allatum, both of which increase oxygen consumption, or for the hydrocortisone-like hormones from the adrenal cortex and a neuro-secretion from the insect brain, both of which increase diuresis. (2) ‘Morphogenetic’ hormones promote growth and cell differentiation. This term can be used for thyroxine when it has a morphogenetic action in stimulating amphibian metamorphosis. The hormone, ecdysone, from the prothoracic gland of some insects is also morphogenetic, since it promotes moulting and ecdysis. There is obviously some overlap between the hormones in these two groups, since no growth can occur without the support of many metabolic processes; but metabolic actions can occur without growth and can usually be examined and described independently. The distinction in terms can help to clarify this.