The times-to-maximum effect of type A botulinum neurotoxin (BoNT/A) depend to some extent on biophysical and biochemical variables. These times range from <3 h (isolated nerve-muscle preparations) to 1-3 weeks (healthy human volunteers). Hydrodynamic equations were solved for different models, some of which produced values of the diffusion coefficient (D) that are comparable to those of other proteins. Other results differed by up to five orders of magnitude. Because the in vivo findings were not related to an unusually small value for D, we propose that there is a protective reservoir for BoNT/A in vivo and possibly for other serotypes.