Epithelial cells from the lens equator differentiate into elongated fiber cells. In the final steps of differentiation, the chromatin appears quite condensed and chromatin breakdown into nucleosomes occurs. DNA breaks due to an endodeoxyribonuclease activity corresponding to at least two polypeptides of 30 and 40 kDa have been identified. To identify the nature and the developmental appearance of initial breaks, nick translation reaction was followed both biochemically and in situ in fiber and epithelial cells from chick embryonic lenses. There is no accumulation of single-strand breaks (SSB) with 3'OH ends in lens fiber cells during embryonic development. Such damage can be increased in these cells by treatment with DNAase I indicating the absence of an inhibitor of the nick translation reaction in fiber cells. However, there are indications of the presence of DNA breaks with blocked termini when the phosphatase activity of nuclease P1 is used. The presence of breaks is also indicated by the large amounts of (ADP-ribose)n found in lens fibers particularly at 11 days of embryonic development (E11) as ADP-ribosyl transferase binds to and is activated by DNA strand breaks. Incubation of lens cells in vitro, which causes nucleosomal fragmentation only in fiber cells, produces SSB with 3'OH ends in both epithelia and fibers. Incubation for short periods, observed in experiments in situ, induces SSB first in the central fiber nuclei, which are late in differentiation. This may indicate that these SSB play a physiological role. Long incubations produce larger numbers of SSB in epithelia than fibers. The SSB in the fibers may have been converted into double-strand breaks (D SB), seen as nucleosomal fragments, and therefore no longer act as substrates for nick translation. The nuclease activity responsible for SSB production is independent of divalent cations and could be implicated in lens terminal differentiation.