In this study we investigated the effect of systemic aminoglycoside administration on the expression of sialoglycoconjugates in the outer hair cell (OHC) glycocalyx of the adult guinea pig. Sialoglycoconjugates were visualized by means of ultrastructural lectin cytochemistry, using Limax flavus agglutinin (LFA) and wheat germ agglutinin (WGA) as probes. Labelling densities were determined for the apical membranes (including the stereocilia and stereociliary cross-links) and basolateral membranes of OHCs in the respective (basal, middle and apical) cochlear turns from animals that had been treated with gentamicin or neomycin for 5 or 15 consecutive days. Our results indicate that: (1) sialoglycoconjugate expression in the OHC glycocalyx demonstrates an intracochlear gradient decreasing towards the apical turn; (2) OHCs demonstrate a polarity in sialoglycoconjugate expression, in that the basolateral membranes contain more sialoglycoconjugates per surface area than the apical membranes; (3) aminoglycoside administration results in reduced expression of sialoglycoconjugates in the OHC glycocalyx; in this respect, basal-turn OHCs are more susceptible than those in the middle and apical turns; (4) reduction in sialoglycoconjugate expression after aminoglycoside administration is more prominent in the basolateral membranes; and (5) the difference in ototoxic potencies between gentamicin and neomycin is not reflected at the level of sialoglycoconjugate expression. The present data support our earlier hypothesis that aminoglycosides, already at an early phase of intoxication, interfere with the function of the endoplasmic reticulum and/or the Golgi apparatus, implying that these organelles play a crucial role in the initial phase of aminoglycoside-induced OHC degeneration.