Turnover of ribulose 1,5-diphosphate carboxylase in barley leaves (Hordeum vulgare L.) was followed over time in light and dark. The enzyme was degraded in prolonged darkness and was resynthesized after the plants were returned to light. Labeling with (14)C showed that simultaneous synthesis and degradation (turnover) did not occur in light. In contrast, the remaining soluble protein was turned over rapidly in light. Although ribulose 1,5-diP carboxylase can be both degraded and synthesized, these processes seem not to occur simultaneously, but can be induced independently by changing environmental conditions.