Arabogalactane sulphate of quinidine (AGSQ) is a slow release preparation of quinidine. The aim of this study was to compare the plasma levels of quinidine obtained by different preparations of AGSQ (AGSQ I, II and III) and to determine which was best suited to therapeutics. The "in vitro" study showed different amounts of quinidine liberated in 6 hours, 34% with AGSQ I, 58% with AGSQ II and 100% with AGSQ III. The plasma quinidine levels were studied after administration of a dose corresponding to 330 mg quinidine base, morning and evening for 7 consecutive days to 27 hospitalised patients; 7 received AGSQ I, 11 received AGSQ II 5, received AGSQ III and 4 quinidine sulphate. The delay in reaching a steady state was 24 hours for the quinidine sulphate 36 hours for AGSQ I, 48 hours for AGSQ II and 60 hours for AGSQ III. The average plasma level on the 7th day (Cee) was 2.74 +/- 0.71 microgram/ml, 2.62 +/- 0.74 microgram/ml and 3.29 +/- 0.72 microgram/ml respectively. The plasma quinidine levels were maintained between toxic and therapeutic levels (3,5 and 1,7 microgram/ml) only with AGSQ II by suppressing the peak observed 1 hour administration of quinidine sulphate. An excellent correlation (r = 0,984) was observed between the plasma quinidine 6 hours after ingestion and the Cee. A blood test during the steady state, 6 hours after ingestion of the drug, is useful in adjusting the dosage. These results suggest that AGSQ II is the preparation best suited for therapeutic usage although it does not give the best relative bioavailability of the drug.