In the brine shrimp, long-term (several days) osmotic and ionic regulation is controlled principally by the activity of Na'K'ATPase. However, short-term cell volume regulation in Artemia and most euryhaline crustaceans is not understood. In this study, Artemia stage 1 nauplii reared at 32 ppt salinity and exposed to hypoosmotic conditions (12 or 4 ppt) exhibited an immediate increase in the activity of ornithine decarboxylase, the rate-limiting enzyme in polyamine synthesis. ODC activity increased within 15 min relative to control and reached a peak within 2 h r of hypoosmotic exposure. The level of increased enzyme activity was directly related to the degree of hypoosmotic exposure (a three-and sevenfold increase in activity at 12 and 4 ppt salinity, respectively). This increase in activity was apparently due to the synthesis of a new enzyme, based on cycloheximide administration. Activity decreased significantly by 4 hr but remained above that of the 32 ppt control throughout the 24-hr exposure period. Levels of putrescine increased significantly within 2 h r in organisms exposed to 4 ppt; spermidine and spermine levels were not affected. When nauplii were exposed to altered ion andor osmotic concentrations it was determined that decreased total osmotic concentration was the proximate factor responsible for ODC induction. However, reduced ion concentration resulted in the inductiodactivation of DFMO-insensitive ODC activity, suggesting the production of a novel enzyme in polyamine metabolism during hypotonic exposure. Changes in ODC activity and polyamines preceded acclimatory changes normally associated with salinity acclimation, further suggesting that modulation of ODC activity and subsequent polyamines synthesis, degradation, andlor transport are mechanistically related t o short-term regulation of osmotic and ionic balance.