There is a widely held misconception that low-molecular-weight polyethylene glycols are "highly lipophilic" permeability probes and therefore are transported across lipid cell membranes. The relative lipophilicity of polyethylene glycols 400 and 600 were examined by determining their partition coefficients (Kd) in water and organic solvents of increasing relative polarity. The Kd of polyethylene glycol 414 between hexane and water was 0.000015, indicating that there are only 1.5 parts of polyethylene glycol 414 in hexane for 100,000 parts of polyethylene glycol 414 in water. When the Kd was determined in organic solvents with increasing relative polarity or "water character", there was a linear increase in Kd. The relative urinary recovery of individual molecular weight fractions of polyethylene glycol 400 in normal volunteers was analyzed. After oral ingestion, there was a progressive decrease in relative urinary recovery of increasing molecular weight fractions of polyethylene glycol 400 suggesting that increase in the molecular size limited polyethylene glycol intestinal permeability. There was excellent correlation between the relative urinary recovery and the hydrophilicity of the intravenously administered polyethylene glycol 400 fractions. It is concluded that polyethylene glycols 400 and 600 are strongly hydrophilic. Since partitioning of polyethylene glycol into lipid phase is negligible in lipid/water mixtures, they are unlikely to be transported via lipid pathways. The intestinal permeability of polyethylene glycols are governed by their molecular size, and once in circulation their urinary excretion appears to be governed in part by their plasma or water solubility.