Ornithine decarboxylase (ODC) activity is known to be strongly enhanced in mammalian cells by a sudden reduction in ambient osmolality. The effect of hypoosmotic shock on the regulation of ODC protein and mRNA levels was studied in a variant L1210 mouse leukemia cell line (D-R cells) which expresses ODC at greater than or equal to 100-fold higher levels than the parental cells. Hypoosmotic stress increased ODC activity in proportion with the osmotic gradient imposed to both D-R cells and their normal counterparts. A 60% decrease in medium osmolality increased ODC activity and the amount of immunoreactive ODC protein from 20- to 30-fold after 4 h without any detectable change in ODC mRNA contents in D-R cells. ODC induction was sustained up to 48 h after hypoosmotic shock, with maximal activity levels being observed at 24 h. Hypotonic shock dramatically increased (up to 36-fold) the rate of ODC synthesis as measured by 10-min pulses with 35S-labeled methionine, in agreement with kinetic constants predicted from the changes observed for the enzyme activity. Moreover, hypoosmotic stress extended the half-life of ODC activity from 35 +/- 10 to 212 +/- 67 min and blocked any degradation of the radiolabeled immunoreactive protein, which had a half-life of 28 +/- 6 min under isotonic conditions, for at least 120 min after addition of cycloheximide. The induction of ODC by hypoosmotic stress was quickly reversed by a sudden upshift of osmolality through a very rapid inhibition of ODC biosynthesis and an increase in the rate of enzyme degradation. Thus, hypoosmotic stress activates the expression of ODC exclusively through post-transcriptional mechanisms in D-R cells. The osmotically induced accumulation of ODC molecules is quite unique as shown by the fact that ODC is the major protein (approximately 25% of total) synthesized during the first 4 h following a 60% hypotonic shock, despite a 30-50% reduction of the rate of labeled precursor incorporation into soluble proteins.