Renal function, fluoride formation and excretion were studied during and after enflurane anaesthesia in seven patients. During anaesthesia, urine flow rate, inulin clearance, PAH clearance and fractional sodium excretion were 13, 78, 65 and 49% of control values, respectively. Renal function was promptly restored postoperatively. Enflurane was metabolized to inorganic fluoride with a mean maximal serum level of 17.4 +/- 3.3 microM. Fractional fluoride excretion decreased during anaesthesia to 35% of the control value. Postoperatively, there was a highly significant correlation between the increase in fractional fluoride excretion and the rise in urinary pH during two consecutive 3-hour periods. It is suggested that tubular reabsorption of fluoride is inversely related to tubular fluid pH and that fluoride is reabsorbed by non-ionic diffusion.