Interest and information concerning the biological roles of taurine have greatly increased during the last 20 years. This simple aminosulfonic acid has been shown to be an essential nutrient for cats, and probably also for primates, especially during development. A dietary deficiency of taurine in cats results in malfunctioning of a number of systems, including the visual system in which decreases in visual acuity, electroretinogram, and visual evoked responses occur along with degeneration of the retina and tapetum lucidum, the biological mirror behind the retina (16, 33, 34, 49, 50,148-151,153,192,277, 374, 427, 430, 483); the reproductive system in which spontaneous resorptions, abortions, and stillbirth are frequent, and surviving offspring have an abnormal ontogeny (185,315,316,409,413,417-419); the immune system in which the proportions and functions of leukocytes are changed and the morphology and function of the spleen is altered (377, 378); and the cardiovascular system in which myocardial failure (dilated cardiomyopathy) occurs as defined by echocardiographically measured chamber enlargement and thinning of the heart wall (293, 338-340). All of these changes are accompanied by decreased concentrations of taurine throughout the body. Before the association of taurine with these important functions, which was initiated by the landmark discovery of Hayes et al. in 1975 (150), there had still been considerable interest in the physiological actions of taurine, including its role in development. The physiological actions of taurine have been extensively reviewed by two comprehensive papers in this journal in 1968 (194) and 1992 (178), and the actions of taurine in the mammalian central nervous system in 1989 (177), so this review concentrat. es only on that information on taurine that may be relevant to developmental processes.