Ninety-five of 103 episodes of acute rejection encountered in recipients of 112 renal allografts were halted with one to four intravenous "pulses" of methylprednisolone sodium succinate, 30 mg/kg. Cumulative transplant and recipient survival improved after introduction of pulse therapy. No serious complications were observed from this treatment. With gas chromatography, the serum half-life of methylprednisolone was determined to be 2.30 ± 0.70 hours in humans. The peak serum concentration of prednisolone was lower, and the duration of high serum concentration longer after oral administration of prednisone, 30 mg/kg. Intravenous administration of massive doses of methylprednisolone every 48 to 72 hours reversed or halted 92% of acute rejection episodes, did not cause any obvious acute or chronic morbidity, and provided a transient high peak concentration of methylprednisolone alternating with longer periods of low serum levels.