Circadian patterns of spore discharge have been studied in 19 genera of Hymenomycetes in the field at various locations in Kansas, Colorado and Michigan. Spores were collected hourly with portable units of Kramer-Collins Spore Samplers. Relative humidity, availability of adequate soil moisture, temperature and light, under certain conditions and with certain species, are thought to have some influence on the patterns of spore discharge. Several distinct circadian patterns of spore discharge were found. In a number of studies of small forms of the genera Mycena, Inocybe and Psilocybe, where the basidiocarps were growing in a protected situation and the microenvironment remained relatively constant, spore discharge was continuous with little variation throughout the 24-hr period. Numberous species of Cortinarius, Lactarius, Collaybia, Laccaria, Crepidotus, Panacolus and Oudemansiella displayed a pattern of maxinum spore discharge during the mid-part of the night with a minimum during the daytime. That pattern seemed to correlate with the usual fluctuations of rising alternating light and dark. In the gnera Leccinum, Tylopilus, Suillus and Boletus of the Boletales, a similar pattern was found with the exception that peak spore discharge often began much earlier in the day and continued for only a short time into the night. A variation from the pattern of nighttime maxima was found to occur when young sporocarps (species of Lactarius, Collybia and Boletus) were subjected to a somewhat rapidly drying environment for several days that prevented their reaching maturity. However, if it rained before severe desiccation occurred, the basidiocarps revived, grew rapidly and released a single crop of basidiospores. In some cases those same species when found developing under more suitable conditions, discharged their spores in a pattern of maxima at night and minima at day. In several studies of the genera Ganoderma and Poria, spores were released in a pattern with double peaks, one occurring about 6 am and other about 6 pm.