Plasma membranes isolated from chick embryo neurons after 2 days (division phase) and 5 days (maturation phase) in culture showed no differences in the phospholipid (PL) pattern or in the plasma membrane average fluidity. Significant ganglioside (Ggl) variations were only found in GM1 and GD3 which represent 35% of the total plasma membrane N-acetylneuraminic acid (NeuNac) content (1.8-fold GM1 NeuNac increase, 1.4-fold GD3 NeuNac reduction). Lactoperoxidase-catalyzed radioiodination (¹²⁵I) under saturating conditions indicated that the hydrophobic core of cell surface Ggl was more exposed to the hydrophilic extracellular environment than that of PL. Nevertheless, when interneuronal contacts start to form, the ¹²⁵I-labelling of both PL and Ggl dropped progressively throughout the maturation phase. Further analysis revealed that (1) plasma membrane GD3 was not accessible to the enzymatic labelling, and (2) surface phosphatidylethanolamine (PE) and GT1b became more accessible to the external medium during development, whereas the opposite was found for phosphatidylcholine (PC) and GM1. The overall profile of radioiodinated PL and Ggl was unaltered when poly-L-lysine was substituted for polyethyleneimine as culture substratum, except for an increase in GT1b labelling 24 h after plating. These results suggest the existense of a ''buffering'' mechanism protecting the neuronal plasma membrane during cellular growth from marked changes in fluidity, polar lipid composition and relative localization of polar lipids with regard to the extracellular environment. In addition, our findings suggest that: (1) GD3, which is at maximal levels during the division phase, is not exposed to the extracellular environment; (2) PE, PC, GM1 and GT1b undergo subtle topological reorganization as cellular maturation proceeds, and (3) the culture substratum may modulate the exposure of certain cell surface glycolipid components.