During terminal differentiation, lens fiber cells increase in length by 100-1000 fold. The mechanism of cellular elongation is not well understood but previous measurements on differentiating embryonic fiber cells indicated that cell volume increases in direct proportion to cell length. Fiber cells are tightly packed within the lens. Under these circumstances, a cellular volume increase might be transduced into an increase in length. Such considerations have led to the suggestion that volume increase may be the driving force for cell elongation. In the present study, we tested this model using a strain of mice in which a GFP transgene is expressed sporadically in the lens. We employed a combination of confocal microscopy, image deconvolution, and volume-rendering techniques to visualize the structure of individual GFP-expressing cells within the living lens. We examined their morphology at various stages of differentiation; from the central epithelium to young fiber cells. At each stage, cell length and volume were determined. In contrast to earlier studies, our data indicated that increases in cell length were not matched by a proportional increase in volume. Rather, the initial phase of elongation (in which cells increased in length from <10 to >150 microm) appeared to result largely from a change in cell shape. The present observations suggest that the driving force for elongation of primary and secondary fiber cells may differ fundamentally.