Nitric oxide (NO) has emerged as a fundamental signal molecule involved in the responses of plant to stress. A role for NO in the regulation of methionine sulfoxide reductase (MSR) mRNA expression and high light acclimation was studied in a green macroalga Ulva fasciata Delile. Transfer from darkness to high light (≥1,200 μmol photons m(-2) s(-1)) inhibited photosynthesis and growth but increased NO production and UfMSRA and UfMSRB transcripts. Treatment with an NO scavenger, 2-(4-carboxy- phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (cPTIO), at 1,200 μmol photons m(-2) s(-1) caused a further growth inhibition accompanied by an inhibition of the increase of UfMSRA and UfMSRB transcripts by high light, while treatment with an NO generator, sodium nitroprusside (SNP), alleviated the growth inhibition and enhanced UfMSRA and UfMSRB expression. Exposure to moderate light (300 μmol photons m(-2) s(-1)) conditions also increased UfMSRA and UfMSRB transcripts, which were not affected by cPTIO treatment but were enhanced by SNP treatment. So, NO does not mediate the up-regulation of UfMSR genes by transfer to moderate light possibly as a precautionary mechanism in the sense of increasing light intensities in the daytime. In conclusion, NO production can be induced in U. fasciata upon exposure to high light for up-regulation of UfMSRA and UfMSRB expression but the level of NO production is not sufficient for acquisition of full tolerance to high light stress. Enhanced NO production by an exogenously applied NO generator can effectively trigger the high light acclimation process, including UfMSRA and UfMSRB expression.