Feulgen/DNA cytophotometric determinations were carried out on early prophases in the meristems of seedlings obtained by germinating seeds of different accessions of Festuca arundinacea at 10°C, 20°C, or 30°C. Feulgen/DNA contents increased significantly with the increase in the temperature of seed germination. In each accession, the greater the increase in absorption in seedlings obtained at 30°C, the lower the absorption in seedlings obtained at 10°C. In contrast, Feulgen/DNA contents did not undergo changes when the temperature was altered at developmental stages other than seed germination. The results of molecular hybridizations (slot blots) indicated that the redundancy of repeated DNA sequences belonging to two families having Cot ranges of 0–2×10-1 and 2×10-1 −2×100, respectively, was significantly higher in the genome of seedlings obtained at 30°C than in that of seedlings obtained at 10°C. When centrifuged to equilibrium in CsCl density gradients, the DNA extracted from seedlings obtained at 30°C formed a heavier and a lighter shoulder with buoyant densities of 1.707 g/ml and 1.692 g/ml, respectively, in addition to the main band (1.701 g/ml). Only a less apparent shoulder banding at 1.706 g/ml was formed by the DNA extracted from seedlings obtained at 10°C. After seed germination in the presence of [3H]-thymidine for 24 h at 30°C, most of radioactivity was found in the guanine + cytosine- or adenine+thymine-enriched DNA fractions, which formed the two shoulders in the density profile. In contrast, only guanine+cytosine-enriched fractions, which formed the heavier shoulder, were preferentially labelled in the DNA from seedlings obtained at 10°C. These results prove that fluid domains do exist in the nuclear DNA of F. arundinacea. These DNA domains are capable of rapid, quantitative alterations, which represent the direct responses of the genome to developmental and environmental stimuli. Seed germination appears to be a limited, specific period in development within which the adaptive response to temperature variations can be put into effect.