The role of cyclic adenosine monophosphate (cAMP) during growth and development of Aspergillus nidulans was investigated. In normal cultures the highest amount of cAMP, expressed on a dry weight basis, was found after 24 h of growth when still more than 5% glucose was present in the medium. After depletion of the medium even a slight fall in cAMP was noted. Glucose concentrations ranging from 0.5-12% resulted in a slight decrease in the amount of cAMP as measured after 24 h of growth. Cultures with manganese deficiency resulted in a low cAMP level after 24 h of growth. However, the exhaustion of glucose in the absence of manganese was connected with a sharp increase in cAMP. This indicates that manganese shortage was not a direct cause of the low cAMP level after 24 h. The amount of cAMP rose with increasing concentration of manganese in the medium until a maximum at 0.25 muM. It is tempting to speculate that this rise in cAMP in the manganese deficient culture is explained by the absence of glucose, that in the control culture is derived from the breakdown of the reserve material alpha-1,3-glucan. Addition of manganese after glucose exhaustion to a manganese deficient culture induced cleistothecium formation. However, they contained only a few ascospores indicating the importance of alpha-1,3-glucan as a carbon and energy source for ascospore formation. The regulation of the level of cAMP by the transport of glucose into the cell or its intracellular concentration is discussed.