Disposition of oral neomycin in calves was determined using 14C-labeled neomycin. The influences of age, diet, and method of administration were observed. All calves were killed 96 h after a single oral dose of [14C]neomycin (approximately 30 mg/kg) and the distribution of 14C in excreta and tissues was determined. As indicated by urinary excretion, absorption of neomycin was greater in 3-d-old calves (11.1 +/- 1.8% of the dose) than in 54- to 64-d-old nonruminating calves (1.5 +/- .58% of the dose) dosed similarly. Absorption of neomycin was similar in nonruminating (1.5 +/- .58%) and ruminating (2.13 +/- .62%) calves when the doses were administered in solution via a nippled bottle. In ruminating calves, absorption was somewhat less when the dose was administered on feed via a gelatin capsule (.5 +/- .06% of the dose) than when given in solution via a nippled bottle (2.13 +/- .62% of the dose). In calves dosed at 3 d of age, 14C concentration in the kidneys represented 55 +/- 4.9 ppm of neomycin equivalents. The next highest concentration occurred in the livers, which contained less than 5% of the level in kidneys. Tissue concentrations of 14C were related to absorption (as indicated by urinary excretion). Isolation and characterization (positive-ion fast atom bombardment mass spectroscopy and nuclear magnetic resonance spectroscopy) of 14C compounds in kidneys of calves dosed at 3 d of age indicated that at least 90% of the 14C was present as neomycin. Neomycin was also the major 14C compound in feces of all calves (70 to 80% of the 14C present).(ABSTRACT TRUNCATED AT 250 WORDS)