1. The effects of acclimation to constant and diurnally cycling temperatures upon water-electrolyte regulation were examined in goldfish held at 20, 25, 30 and 25 +/- 5 degrees C, and sampled at 03.00, 09.00 15.00 and 21.00 h. Plasma and epaxial muscle levels of Na, K, Ca, Mg, Cl and water were determined. Using Cl space as an indicator of extracellular phase volume, mean cellular cation concentrations were estimated. 2. Fish held at constant temperature exhibited significant diurnal variations in all ions except plasma magnesium and muscle potassium. With the exception of muscle choride, however, the occurrence of peak and/or minimum concentrations tended to be inconsistent in relation to specific sampling times. Somewhat more regularity was apparent in terms of dark or light periods. 3. Under constant temperature conditions plasma Cl increased with increasing temperature, while Na declined. Plasma magnesium was consistently higher at 25 degrees C than at either 20 or 30 degrees C. This was true of muscle Mg as well and, generally, also of Na, K and Cl. Water content tended to rise at higher temperatures in these animals, as did cellular phase volume, while extracellular phase volume was reduced. 4. Exposure to cycling temperatures was associated with a number of significant departures from the pattern seen at constant temperature. Ionic concentrations tended to be lower. By comparison with animals sampled at constant temperature and comparable times, diurnal stability was greater. In several instances (e.g. muscle Cl- and K+, cellular and extracellular phase volumes) variations with temperature were significantly different. This was also the case with ion pairs such as K and Na, and Ca and Mg. 5. These observations raise obvious questions regarding the validity of earlier descriptions of water-electrolyte status in species normally exposed to fluctuating temperatures. The variations seen under cycling temperature conditions, however, appeared to be adaptively appropriate. Reductions in plasma ion levels, for example, would tend to reduce costs of ionic regulation. The stability of plasma: cellular K concentrations should desensitize muscular excitability in relation to changing temperature conditions. This was also true of cellular levels of generally stimulating (Mg, K) and generally inhibitory ions (Ca, Na) known to influence metabolic processes.