Nitric oxide synthase (NOS) type II is induced in many cell types in response to cytokines or endotoxin. The duration of type II NOS mRNA expression in astroglial cells and macrophages in vitro is brief, even in the continuous presence of inducers, and their removal dramatically accelerates mRNA decay. Addition of cycloheximide, in the presence or absence of actinomycin D, protected the mRNA from degradation. Whereas type II NOS mRNA was partially stabilized by actinomycin D, manganese superoxide dismutase mRNA was almost completely stabilized. This suggests that type II NOS mRNA stability is regulated via transcription- as well as translation-dependent processes and that the effect of actinomycin D is mRNA specific.