The renal effects of parathyroid hormone (PTH) and dibutyryl 3'5'-cyclic AMP (DBcAMP) were studied in thyroparathyroidectomized hamsters. The hamsters were permitted free access to food and water or fasted for 16 h. PTH caused a phosphaturia in the fed hamster (fractional excretion of phosphate (FEPO4) increased from 5.8 +/- 1.3 to 27.4 +/- 4.6%, P less than 0.001) but not in the fasted hamster (from 9.9 +/- 2.5 to 12.4 +/- 2.5%, NS), whereas calcium excretion decreased significantly in both groups. There was no significant difference in blood acid-base or phosphate levels between the two groups. Insulin did not restore the phosphaturic response to PTH (FEPO4 from 7.7 +/- 2.6 to 5.3 +/- 1.7%), whereas phosphate or NH4Cl infusion did, FEPO4 increasing from 20.9 +/- 3.1 to 38.1 +/- 5.4% (P less than 0.02) and from 19.5 +/- 3.8 to 39.0 +/- 7.5%, respectively. DBcAMP caused a phosphaturia both in the fasted (from 9.6 +/- 2.7 to 20.1 +/- 4.5%, P less than 0.01) and fed (from 2.5 +/- 0.5 to 10.7 +/- 1.5%, P less than 0.02) hamster. A fasting state of up to 64 h did not produce resistance to PTH in the rat. It is concluded that fasting produces resistance to the phosphaturic but not the calcium-retaining effects of PTH in the hamster. The resistance may occur, at least partly, prior to the production of cAMP within the renal tubular cell.