Meiosis in Coprinus lagopus is normally initiated at night under normal diurnal conditions. This timing can be shifted to 0900-1000 h by using a 16h light-8h dark regime with the light cycle commencing at 1600 h. For the initiation of meiosis, C. lagopus is temperature sensitive (35 C) under continuous light. The temperature sensitivity can be overcome by a dark period. The sensitive period occurs within 7 h of karyogmay. Only the first 2 h, however, are sensitive to light and high temperature. A shift-up to restrictive conditions before the sensitive stage effectively arrests the process leading to karyogamy. The arrest is reversible within 20 h. Upon returning to a 25 C chamber, karyogamy begins in 6 h in all basidiocarps. Thus using this technique accurate control of the initiation of meiosis can be achieved. Howeevr, prolonged arrest beyond 16 h causes the basidia to revert to mitosis and mycelial growth. The nucleolus is reduced in size and exhibits a large vacuole and a lack of granular components as shown by electron microscopy.