Marine flatfishes have a low metabolic rate and routinely encounter large fluctuations in salinity, and are therefore of interest in the study of diffusive water flux (a proxy for transcellular water permeability), oxygen consumption (ṀO₂), ammonia excretion and urea-N excretion as a function of salinity and seawater [Ca²⁺]. These parameters were measured in two coastal marine flatfishes, Pacific sanddab and Rock sole acclimated to 31 ppt and exposed acutely (for up to 3 h), to environmentally relevant salinities of 45, 15.5, or 7.5 ppt. In both species, diffusive water flux and ammonia excretion rates increased as salinity decreased. ṀO₂ and urea-N excretion rates remained relatively unchanged. Nitrogen quotient analysis indicated increased oxidation of protein at lower salinity. A second experimental series was performed on Rock sole to separate the effects of salinity from those of ambient [Ca²⁺]. In direct contrast to the significant increase seen at 7.5 ppt, reducing salinity from 31 ppt to 7.5 ppt while maintaining [Ca²⁺] at 10 mM or increasing it to 20 mM resulted in no change in diffusive water flux rate, demonstrating that reduced [Ca²⁺], rather than reduced salinity itself, is the primary cause for the increases in diffusive water flux. However, ammonia excretion rate increased when salinity was decreased and [Ca²⁺] was increased compared to 31 ppt with added [Ca²⁺]. Our results demonstrate that both diffusive water flux and ammonia excretion rates are a function of salinity, that neither are coupled to ṀO₂, and that ambient [Ca²⁺] also plays a role in these rates.