The effects of adding various macromolecular substances to 2% prilocaine on duration of rat infraorbital nerve block were investigated. The tested substances consisted of dextrans with lipophilic or charged substituents as well as other neutral or highly charged macromolecules. Most of the adjuvants caused significant prolongations of sensory block. For substituted dextrans the duration of sensory block degree 3 amounted to between 120% (3% capryldextran II) and 350% (3% carboxymethyldextran) in comparison to prilocaine plain. The corresponding values for hydroxypropylstarch (3%) alginic acid (0.5%), beta-cyclodextrin (1.5%) and hyaluronic acid (0.25%) were about 170%, 285% and 380%, respectively. The results suggest that the increased duration of local analgesia by prilocaine is related to increased viscosity of the solution produced by the macromolecular compounds. The mechanism seems to be of a physical character, and hyaluronic acid seems to be worthy of further studies.