Addition of cyclic AMP to specific sites in plate cultures of a mature dikaryot ic colon y of Coprinus cinereus caused a twofold increase in the amount of glycogen over the whole culture within 19 h. There was no evidence for non -random distr ibution of glycogen around the site of admin istration of th e cAM P . In th e normally developing fru it body levels of endogenous cAMP were considerably elevated in frui t body initials and remained unusually high in the frui t body until very late stages of development, when th ey declin ed to about the concentration s observed in mycelium. These cAMP accumulations were positively correlated with localized accumulations of glycogen within the fruit body. Published data from in vitro ana lyses suggest that cAMP may inhibit glycogen synthesis and activate glycogen breakdown in Coprinus. It is suggested that observations reported here would be consistent with this view if cAMP were specifically involved in th e bulk translocat ion of glycogen, i.e. promoted glycogen breakdown in order to mob ilize carbohydrate for tran sport , rather than immediate metabolism .