AbstractBrine shrimp nauplii challenged with artificial sea water containing 2.5 M NaCl maintain significantly higher levels of cytoplasmic malate dehydrogenase (s‐MDH) than larvae incubated in sea water having 0.5 M NaCl. Eight to ten hours after emergence, free‐swimming nauplii living in 0.5 M NaCl exhibit a steady decline of s‐MDH for 20–40 hours; the decrease is less and stabilizes earlier — in nauplii incubated in 2.5 M NaCl. The14C‐labeled amino acids produced by H14CO3fixation were rapidly incorporated into newly formed s‐MDH protein as assayed using quantitative rocket immunoelectrophoresis (IEP) with monospecific antiserum prepared against purified brine shrimp s‐MDH. Higher rates of enzyme biosynthesis (>45%) occurred in 2.5 M NaCl together with rapid s‐MDH turnover (half‐life = 17 hours), accounting for the difference observed in enzyme level between different salt regimes. In contrast, incorporation of14C‐labeled amino acids into total cytoplasmic protein decreases slightly in high salt, suggesting that a preferential synthesis of s‐MDH is taking place. Temporal patterns of s‐MDH during embryonic development were monitored using both catalytic activity and quantitative IEP assays. Levels of s‐MDH seen in encysted gastrulae (0.22 units or 0.57μg s‐MDH protein/100 embryos) remain relatively constant through the E1and E2emergent stages until the exhibited decline observed in the naupliar stage. The results are discussed in relation to the bioenergetics and temporal development of water and ele