The levels of ceruloplasmin (Cp) and total protein were measured in serum and urine from rats with experimental nephrotic syndrome (NS) for 20 days after a single injection of puromycin aminonucleoside (PAN). Control values for Cp in serum and urine were: 0.23 +/- 0.01 mg/ml and 0.063 mg/day, and those for total protein were: 7.1 +/- 0.14 g/dl and 2.1 +/- 0.6 mg/day, respectively. It was found that: a) serum Cp decreased on day 6, remained low until day 10 (0.07 +/- 0.01 to 0.08 +/- 0.01 mg/ml), and returned to control levels on day 12; b) total serum protein decreased on day 4, reached the lowest value on day 6 (3.3 +/- 0.3 g/dl) and returned to control levels on day 16; c) Cp in urine increased on day 6, reached a peak value on day 8 (0.62 +/- 0.07 mg/day), then decreased but remained high on day 20; and d) total protein in urine increased on day 5, reached a peak value on day 10 (692 +/- 59 mg/day) and returned to control values on day 20. There was a close association between the levels of Cp and total protein in serum and urine. The decrease of circulating Cp indicates that a typical acute phase response does not occur in the PAN-nephrotic rats. The loss of Cp in the urine of the nephrotic rats may contribute to the decrease in the serum levels of this protein.