Feulgen hydrolysis curves were plotted for isolated nuclei from various regions (ecto-, meso- and endoderm) and developmental stages [mid to late gastrula (Harrison-St. 12b) and tailbud stage (Harr. St. 25, 35)] ofTriturus vulgaris embryos. Hydrolysis was carried out for 10 to 70 min with 5N HCl at 20° C. Single-peaked hydrolysis curves with a maximum between 30 and 40 min were obtained in all cases. The ascending limbs of all the hydrolysis curves are similar in shape up to the maximum. Differences in DNA content of nuclei from different embryonic regions become visible not only at the optimum time but also after short hydrolysis times (10 min). After reaching the maximum there are significant differences in the descending parts of the curves. In the endoderm and mesoderm a drastic loss of apurinic acid occurs after 40 and 50 min of hydrolysis, respectively, whereas in the nuclei of the mesencephalon and the neurectoderm no measurable extraction of depolymerized DNA can be detected after 60 min. These differences in chromatin stability make lead to a levelling out of the Feulgen values over longer hydrolysis periods, so that differences in the DNA content of nuclei from different parts of the embryo can no longer be detected after 60 min. Nuclear volume and histone content have no influence on the results obtained from the Feulgen reaction when optimum hydrolysis times are used.