The effects of glucose and parathyroid hormone (PTH) on the transport and metabolism of myoinositol (MI) and [2-3H]MI were studied in isolated perfused dog kidneys. Studies during perfusion of kidneys with normal and elevated glucose concentrations demonstrated that under normal conditions the isolated kidney reabsorbed 94.7 ± 0.2% of the filtered MI, and the renal production of 3H-metabolities of MI was 117.9 ± 6% of the filtered mI load. This indicated that entry of MI into tubular cells by reabsorption was not the sole pathway for entry into the pool of MI within the kidney undergoing catabolism. High glucose perfusate decreased MI reabsorption to 68.6 ± 4.7% and thus decreased delivery of [2-3H]MI into the catabolic pool from the reabsorptive pathway. In the high glucose experiments, the rate of [2-3H]MI catabolism exceeded [2-3H]MI reabsorption by the same fraction as in normal glucose experiments, which indicates that high glucose did not affect nonreabsorptive access of MI to the catabolic site. In contrast to the effects of glucose, PTH administration resulted in an increase in perfusate MI concentration and a decrease in the perfusate [2-3H]MI specific activity. Concomitantly, urinary MI and [2-3H]MI concentrations were increased, again with a decrease in [2-3H]MI specific activity. These results indicate that PTH caused a release of MI into the urine (not the same as decreased MI reabsorption, which would not affect urinary [3H]MI specific activity) and into the perfusate of the isolated kidneys. These effects on MI release were about coincidental with the increase in urinary cyclic 3',5'-AMP after PTH and preceded the peak phosphaturic effect of PTH. There was no detectable effect of PTH on MI synthesis from glucose as a source of the MI released into the urine and perfusate. However, PTH temporarily halted accumulation of tritiated MI catabolites. There was no effect of inactivated PTH on urinary cyclic 3',5'-AMP or on MI transport, which indicates that the PTH effect on MI handling was a specific hormonal effect. These studies clarify the renal metabolism of MI, and they demonstrate heretofore unknown effects of PTH on the renal handling and metabolism of MI. The effects of PTH on renal MI metabolism have important implications in renal carbohydrate metabolism and phospholipid turnover.