One hundred forty-one free-ranging white rhinoceros (Ceratotherium simum simum) of various ages were immobilized in 1991 (n = 71) and 1992 (n = 70), and the majority were dehorned as part of a conservation program to prevent poaching. Twenty-three animals were darted by personnel on foot and 118 were darted from a helicopter. Fifty-six adult animals were immobilized using a mean (±SEM) dose per animal of 4.2 ± 0.11 mg etorphine combined with 123 ± 4.7 mg xylazine, 13 adult animals were immobilized using a mean dose of 2.03 ± 0.06 mg etorphine and 29.2 ± 0.8 mg fentanyl, 60 adult animals were immobilized with a mean dose of 3.9 ± 0.15 mg etorphine and 13.1 ± 0.43 mg detomidine, and 12 animals (subadults and calves) were immobilized with a mean dose of 1.16 ± 0.28 mg etorphine alone. Hyaluronidase (1,500 IU) was mixed with all drug combinations. The mean induction time for all combinations was 6.4 ± 0.37 min (median = 5 min), with no significant differences in induction times among animals injected with the various drug combinations. The mean duration of immobilization was 38 ± 1.7 min. In 1991, mean reversal time following the administration of naloxone (i.v., 64 ± 4 mg, n = 56) and diprenorphine (i.m., 12.4 ± 0.7 mg, n = 54) was 93 ± 7 sec, and when naltrexone became available in 1992, mean reversal time was 92 ± 5 sec (i.v., 70 ± 2 mg, n = 68). The etorphine/detomidine combination produced significantly less muscle damage, based on creatine phosphokinase (IU/L) measurements, than did the other drug combinations. Serum cortisol (μg/dl) values for subadults and calves immobilized were significantly higher than those in adults, indicating increased stress. Pulse rates for those animals immobilized with etorphine/detomidine were significantly lower than those of animals immobilized with the other drug combinations. Prolonged recumbency was associated with hypoxemia, pulse oximetry revealed a saturated blood oxygen (Sao₂) of 40-60% in recumbent rhinoceros, which was improved by ≥20% using 10-20 mg nalorphine or 20-40 mg nalbuphine, administered i.v. Of the 71 animals immobilized in 1991, five animals died (mortality rate = 7%); three of these deaths were most likely associated with hypoxemia and cardiovascular system collapse. There were no direct capture-related mortalities recorded in the immobilization of 70 animals in 1992 (overall mortality rate for 1991/1992 = 3.4%).