Abstract: Pyracrini phosphas(6701)is a new antimalarial drug developed in China. It possesses the same acridine nucleus as that in quinacrine (A). It had been shown that the toxicity of pamaquine was enhanced by concurrent quinacrine administration, and the maximal plasma pamaquine concentrations were many times higher following quinacrine therapy than the control values. Since both pamaquine and primaquine (PQ) belong to 8-aminoquinoline series, so it is of practical importance to clarify whether there is any effect of pyracrini phosphas on plasma primaquine level following their combined administration.Using a spectrofluorometric method to estimate the plasma primaquine level, the following results were obtained:1. The biological half life of primaquine following concurrent quinacrine administration (A+PQ group) was about twice longer than that following concurrent pyracrini phosphas administration (6701+PQ group), or that after primaquine medication alone.2. The mean plasma primaquine level of A+PQ group is about 10 times higher than that of 6701+PQ group or PQ group at 12 hours after medication.3. No significant difference was found between the plasma primaquine level of 6701+PQ group and that of PQ group.Therefore, it is considered that there is little effect of pyracrini phosphas on plasma primaquine level following combined administration of these drugs to rats.