1. Peripheral blood leucocytes were studied before, and 48 h after acute induction in the rat of either uraemia (by bilateral ureteric ligation or bilateral nephrectomy) or of metabolic acidosis in the absence of uraemia (by ingestion of NH4Cl).
2. A significant, fourfold increase in polymorphonuclear leucocyte count was seen after bilateral ureteric ligation, but no increase occurred in sham-operated animals nor after bilateral nephrectomy, despite equivalent uraemia and acidosis. A lesser degree of polymorphonuclear leucocytes was also seen after unilateral ureteric ligation. A significant but also lesser degree of polymorphonuclear leucocytosis was seen after metabolic acidosis in the absence of uraemia. Correction of acidosis in another group of animals with bilateral ureteric ligation did not prevent polymorphonuclear leucocytosis.
3. Significant lymphopenia, of approximately equal degree, was seen after bilateral nephrectomy, and after metabolic acidosis, but did not occur in control groups.
4. These findings support the hypothesis that polymorphonuclear leucocytosis in acute uraemia or ureteric obstruction may be due to a direct renal mechanism, such as release of a leucopoietin, whereas lymphopenia may be due to metabolic changes associated with uraemia or metabolic acidosis.