The effectiveness of methylene blue (MB) combined with pyrimethamine (PYR), 
chloroquine (CQ) or quinine (Q) was examined in a classical four-day suppressive 
test against a causative agent of rodent malaria, Plasmodium berghei. A marked 
potentiation was observed when MB was administered at a non-curative dose of 15 
mg/kg/day in combination with PYR (0.19 mg/kg/day) or Q (25 mg/kg/day). No 
synergy was found between MB (15 mg/Kg) and CQ (0.75 mg/Kg). Our results suggest 
that the combination of MB with PYR or Q may improve the efficacy of these 
currently used antimalarial drugs.