We have measured glomerular filtration rate (GFR), extracellular fluid volume (ECF), oxalate distribution volume (OxDV), plasma oxalate concentration (POx.), plasma total clearance of oxalate (PCOx.), oxalate metabolic pool size [(OxDV) X (POx.)], renal clearance of oxalate (RCOx.), oxalate excretion, tissue clearance of oxalate (TCOx.) and tissue oxalate accumulation rate [(TOx.A) = (TCOx.) X (POx.)] in three patients with type I primary hyperoxaluria (hyperoxaluria with hyperglycollic aciduria) when they were taking pyridoxine and after discontinuation of the vitamin. Seven days after stopping pyridoxine the plasma oxalate concentration, oxalate metabolic pool size and the urinary excretion of oxalate had all increased between seven- and eight-fold in two of the patients. The third patient showed no changes on stopping pyridoxine. These results support the view that pyridoxine acts by reducing oxalate biosynthesis in some patients with type I primary hyperoxaluria. The possible biochemical basis for this effect is discussed.