Methionine sulfoxide reductase A (MSRA) and MSRB are responsible for the repairing of methionine-R-sulfoxide (Met-S-SO) and methionine-S-sulfoxide (Met-R-SO) back to me-thionine, respectively. Five MSRA isoforms and four MSRB isoforms are discovered in the unicellular green alga Chlamydomonas reinhardtii. Whether high light regulates CrMSR ex-pression via photosynthetic electron transport (PET) was examined. By checking the se-quence of PCR product of each isoform, quantitative real-time primers were designed for discrimination of isoform expression. Light ≥ 300 £gE m-2 s-1 and PET inhibitors inhibited PSII activity (Fv/Fm, Fv´/Fm´) and photosynthetic O2 evolution rate, particularly 1,000 £gE m-2 s-1, in which it did not recover after 3 h. A transfer to dark decreased CrMSRA2, CrMSRA3, CrMSRB1.1, CrMSRB1.2, CrMSRB2.1 mRNA levels but increased CrMSRA4 mRNA levels. When exposed to 50, 300, 600, or 1,000 £gE m-2 s-1, CrMSRA2, CrMSRA3, CrMSRA5, CrMSRB1.1, CrMSRB2.1 and CrMSRB2.2 mRNA levels increased as light ≥ 300 £gE m-2 s-1, and concomitantly CrMSRA4 mRNA level decreased. Changes in mRNA levels increased as light intensity increased. The treatment of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) in 1,000 £gE m-2 s-1 inhibited high light effect, and the treatment of 2,5-dibromo-3-methyl-6- isopropyl-p- benzoquinone (DBMIB) in 50 £gE m-2 s-1 increased CrMSRA3, CrMSRA5 and CrMSRB2.2 mRNA levels but decreased CrMSRA4 mRNA level. The application of phena-zine methosulfate (PMS), an electron donor to P700+ that promotes cyclic electron transport, in 300 £gE m-2 s-1 enhanced the increase of CrMSRA3 and CrMSRA5 mRNA levels by high light but inhibited the decrease of CrMSRA4 mRNA level, reflecting a role of cyclic PET. The above results let us to draw a conclusion that plastoquinone as reduced status mediates the expression of CrMSRA3, CrMSRA4, CrMSRA5 and CrMSRB2.2 by high light. The im-plication of linear electron transport and cyclic electron transport on the regulation of CrMSR gene expression will be discussed.We speculated that the high light up-regulation of CrMSR mRNA expression offers the resistance of Chlamydomonas to photooxidative stress.