The toxic effects of metabolites that are known to accumulate in renal failure are described and the role that they may play in causing uremic symptoms is considered. The opinion of the Authors is that all they are likely to take a lesser or greater part in uremic intoxication. Methylguanidine seems to be very important in this context while for some others (like amines) nothing can be stated for studies on their chronic toxicity are lacking. The hypothesis is also considered of the accumulation of unidentified toxic metabolites with a middle molecular weight. It is stated, as to this problem, that the clinical evidence apparently supporting univocally their existence, is instead also consistent with the hypothesis of toxins (like methylguanidine) having a preferential distribution in the intracellular fluid compartment.