In conidia of Neurospora crassa germinating at 25 degrees C, DNA synthesis measured by incorporation of tritiated adenosine reaches a maximum soon after the outgrowth of the germ tube (6--7 h after inoculation). In conidia heat-treated at 46 degrees C (for 15 h), a maximum of incorporation of the DNA precursor occurs already 1 h after inoculation, then the incorporation progressively declines until the end of the heat-shock. When such conidia are shifted to 25 degrees C, a maximum of DNA synthesis occurs during the development of the presumptive conidiophore as at the outgrowth of normal germ tubes. This wave of DNA synthesis is followed by a second maximum of DNA synthesis, occurring only in the microcyclized cultures, when the premature differentiation of proconidia takes place. Prevention of this second wave of DNA synthesis with hydroxyurea or 5-fluorodeoxyuridine respectively reduces or fully inhibits such induced conidial differentiation.