The Eleusinian Mysteries were one of the most sacred rites of the ancient Greeks. It was forbidden for initiates to speak openly of exactly what transpired during the sacred ritual, and the proceedings remain shrouded in mystery today. However, some hypotheses have been advanced which fit the available data, and some of these ideas have mycotheological significance.
A lengthy and quite convincing argument for the role of entheogens -- and most likely mycologically-based ones -- in the Mysteries was made by Gordon Wasson and others in The Road to Eleusis.
It may be significant that the mythological originator of the Eleusinian mysteries was named Erysibe -- "ergot" (Claviceps purpurea). This is a fungus which grows on wheat, and from which an assortment of medicinal, hallucinogenic, and poisonous substances may be derived.
Robert Graves suggested a different fungal source for the kykeon, a Panaeolus mushroom containing psilocybin and related substances.
Albert Hoffman on entheogens and the mysteries
brief quotations from various sources
a historical review-- note especially the picture of "Demeter and Persephone reunited"
the Hymn to Demeter
another Hymn to Demeter
Baubo -- the female clown of the Mysteries
newsletter of the Program in Bronze Age Archaeology of the U of Manitoba
persistence of the Eleusinian Mysteries in a modern Greek Orthodox church
revival of the Eleusinian Mysteries following the ergot hypothesis
some modern Pagan articles about the Mysteries
and some Neopagan Eleusinian festivals