PEGylation of therapeutic proteins is commonly used to extend half-lives and to reduce immunogenicity. However, reports of antibodies toward PEGylated proteins and of poly(ethylene glycol) (PEG) accumulation suggest that efficacy and safety concerns may arise. To understand the relationship among the pharmacology, immunogenicity, and toxicology of PEGylated proteins, we require knowledge of the disposition and metabolic fate of both the drug and the polymer moieties. The analysis of PEG by standard spectrophotometric or mass spectrometric techniques is problematic. Consequently, we have examined and compared two independent analytical approaches, based on gel electrophoresis and nuclear magnetic resonance (NMR) spectroscopy, to determine the biological fate of a model PEGylated protein, (40K)PEG-insulin, within a rat model. Both immunoblotting with an antibody to PEG and NMR analyses (LOD 0.5 μg/mL for both assays) indicated that the PEG moiety remained detectable for several weeks in both serum and urine following intravenous administration of (40K)PEG-insulin (4 mg/kg). In contrast, Western blotting with anti-insulin IgG indicated that the terminal half-life of the insulin moiety was far shorter than that of the PEG, providing clear evidence of conjugate cleavage. The application of combined analytical techniques in this way thus allows simultaneous independent monitoring of both protein and polymer elements of a PEGylated molecule. These methodologies also provide direct evidence for cleavage and definition of the chemical species present in biological fluids which may have toxicological consequences due to unconjugated PEG accumulation or immunogenic recognition of the uncoupled protein.