Objective To investigate the lethal effect of exogenous nitric oxide on blood-stage Plasmodium vivax in vitro. Methods The immature trophozoites from patient were diluted with human RBC to be a suspension of P. vivax-human RBC at 2% hematocrit and over 0.5% parasitemia. Sodium nitroprusside （SNP）, hemoglobin（Hb）, L-cysteine and FeSO4 were added to the parasite-blood suspension and made the drug final concentration to SNP 0，0.02, 0.05, 0.10, 0.20, 0.50, and 1.00 mmol/L, SNP 1.00 mmol/L+Hb 0.15 mmol/L, SNP 1.00 mmol/L+FeSO4 0.15 mmol/L, SNP 1.00 mmol/L+L-cysteine 1.00 mmol/L, and SNP 1.00 mmol/L+ FeSO4 0.15 mmol/L+ L-cysteine 1.00 mmol/L, respectively. After at least 12 h incubation, the parasites developed to mature schizonts. Parasite maturation was observed in culture by Giemsa staining of samples. The mature schizonts were counted, and the inhibition ratio of exogenous nitric oxide to blood-stage P. vivax was computed. The differences of inhibition ratio in the groups were compared. Results SNP（0.02 mmol/L） was not cytotoxic to blood-stage P. vivax parasites with an inhibition of （0.84±1.69）%. When the concentration of SNP increased to 0.05 mmol/L, the inhibition ratio was （12.26±3.04）% which showed that exogenous nitric oxide released from SNP （≥0.05 mmol/L）killed the blood-stage P. vivax parasites, and the higher the SNP concentration, the larger the inhibition. Addition of hemoglobin, L-cysteine, FeSO4 and L-cysteine with SNP led to a decrease of the inhibition from （85.40±2.90）% to （5.90±2.90）%, （25.86±4.02）%, （30.16±2.75）%, （16.71±2.30）%, respectively. Conclusion Exogenous nitric oxide released from SNP kills blood-stage P. vivax parasites in vitro. Howev-er, hemoglobin, L-cysteine, and FeSO4 can reverse the lethal effect of the parasites.
