Culture filtrates from surface and deep cultures ofAspergillus oryzaewere tested for the presence of stimulatory and inhibitory substances, by examining the effect of the nitrates, when added with the inoculum, on growth rate and maximum yield of stationary cultures of the same organism. Filtrates were prepared from various stages of both large- and small-inoculum cultures by each method of growth.Both stimulatory and inhibitory substances were found. Inhibitory substances predominated in the filtrates from early stages of a small-inoculum culture, more especially when these were derived from a deep culture, which showed a pronounced effect of inoculum size. At other stages of growth of small-inoculum cultures and all stages of growth of large-inoculum cultures tested, stimulatory substances were found.It is emphasized that the stage of production of such substances is of crucial importance. Inhibitory substances acting at an early stage of growth of small-inoculum cultures have an adverse effect on growth of late stages. If, however, stimulatory substances are present in sufficiently high concentration at a sufficiently early age, as in large-inoculum cultures, they exert a favorable influence on later stages of growth. These phenomena explain the effects of inoculum size.