In a recent communication1 we described a method for the determination of guanidine bases in the blood, and noted that certain patients suffering from arterial hypertension showed higher values than did normal persons. Since this publication, the work has been carried forward and a definite improvement in the method has been achieved. The method differs from that previously described in that the guanidine bases are absorbed by blood charcoal from a filtrate made sligtly alkaline, and subsequently are released from the charcoal by means of acidified alcohol. The details of this method have been described elsewhere,2 but a summary of it will be given before discussing the results obtained. METHOD (1) Nitroprusside reagent Stock solution: 10 per cent sodium nitroprusside 10 per cent potassium ferricyanide 10 per cent sodium hydroxide 1 volume 10 per cent sodium nitroprusside 1 volume 10 per cent potassium ferricyanide 1 volume