The sea is the home of two types of animal; primary marine inhabitants of wholly marine ancestry (mainly invertebrates) and secondary marine inhabitants (mainly fish and higher vertebrates) which have had a freshwater or terrestrial phase in their evolutionary history. The former have blood osmolarities close to that of seawater, and only encounter osmotic problems at the sea's margins (e.g. intertidal zones, lagoons, estuaries) where salinities are different from the open sea; the latter have low blood concentrations and potentially have problems of salt loading and water loss in seawater. Most primary marine inhabitants are stenohaline, live in the open sea and encounter no osmotic stress, and can tolerate little change in external salinity. The bulk of their euryhaline relatives living in the more demanding environments of littoral zones and estuaries rely heavily upon behavioural osmotic control. Sessile animals employ isolation responses (burrowing, shell valve closure etc.) when external salinities are stressful; mobile species have been little studied, but some are known to avoid deleterious salinities or choose optimum concentrations. The existence of predictive and reactive categories of behavioural osmotic control in mobile animals is suggested. Intracellular osmotic regulation by changes in the size of the pool of non-essential amino acids is the basis of much penetration of brackish/hypersaline water by primary marine inhabitants; however, it is doubtful whether changes in amino acid concentration are rapid enough to prevent cellular swelling/shrinkage in animals exposed to short-term salinity fluctuations (e.g. in estuaries, splash-zone pools). Extracellular osmoregulation is characteristic of fish, many euryhaline crustacea and a few representatives of other groups. It depends primarily on salt pumps (located usually at gut and/or gill) but low integumentary permeabilities to salt and water are equally essential. The permeability of teleost eggs to salt and water is given special consideration because of their hypo-osmotic state and apparent absence of osmoregulatory structures. In elasmobranchs and coelocanths the problems of salt loading and water loss have effectively been met by separate mechanisms. Osmotic water loss is opposed by high blood concentrations of urea (maintained by renal reabsorption and low branchial permeability) which result in a total blood osmolarity rather higher than that of the external medium.(ABSTRACT TRUNCATED AT 400 WORDS)