Sixteen captive and wild-caught American alligators (Alligator mississippiensis), seven juveniles (≤1 m total length [TL]; 6.75 ± 1.02 kg), and nine adults (≥2 m TL; 36.65 ± 38.85 kg), were successfully anesthetized multiple times (n = 33) with an intramuscular (i.m.) medetomidine–ketamine (MK) combination administered in either the triceps or masseter muscle. The juvenile animals required significantly larger doses of medetomidine (x̄ = 220.1 ± 76.9 μg/kg i.m.) and atipamezole (x̄ = 1,188.5 ± 328.1 μg/kg i.m.) compared with the adults (medetomidine, x̄ = 131.1 ± 19.5 μg/kg i.m.; atipamezole, x̄ = 694.0 ± 101.0 μg/kg i.m.). Juvenile alligators also required higher (statistically insignificant) doses of ketamine (x̄ = 10.0 ± 4.9 mg/kg i.m.) compared with the adult animals (x̄ = 7.5 ± 4.2 mg/kg i.m.). The differences in anesthesia induction times (juveniles, x̄ = 19.6 ± 8.5 min; adults, x̄ = 26.6 ± 17.4 min) and recovery times (juveniles, x̄ = 35.4 ± 22.1 min; adults, x̄ = 37.9 ± 20.2 min) were also not statistically significant. Anesthesia depth was judged by the loss of the righting, biting, corneal and blink, and front or rear toe-pinch withdrawal reflexes. Recovery in the animals was measured by the return of reflexes, open-mouthed hissing, and attempts to high-walk to the opposite end of the pen. Baseline heart rates (HRs) were significantly higher in the juvenile animals (x̄ = 37 ± 4 beats/min) compared with the adults (x̄ = 24 ± 5 bpm). However, RRs (juveniles, x̄ = 8 ± 2 breaths/min; adults, x̄ = 8 ± 2 breaths/min) and body temperatures (juveniles, x̄ = 24.1 ± 1.1°C; adults, x̄ = 25.2 ± 1.2°C) did not differ between the age groups. In both groups, significant HR decreases were recorded within 30–60 min after MK administration. Cardiac arrhythmias (second degree atrio-ventricular block and premature ventricular contractions) were seen in two animals but were not considered life-threatening. Total anesthesia times ranged from 61–250 min after i.m. injection. Although dosages were significantly different between the age groups, MK and atipamezole provided safe, effective, completely reversible anesthesia in alligators. Drug-dosage differences appear to be related to metabolic differences between the two size-classes, requiring more research into metabolic scaling as a method of calculating anesthetic dosages.