To investigate the regulation of the biosynthesis of ergot alkaloids by end products, the effect of exogenous agroclavine-1 and epoxyagroclavine-1 on their accumulation by P. sizovae was studied. The added alkaloids stimulated considerably their own biosynthesis depending on their concentration and time of introduction. The stimulating effect of both alkaloids and products of their degradation was suggested. Exogenous agroclavine-1 and epoxyagroclavine-1 changed the relation between the quantities of intra-and extracellular alkaloids, thus pointing to their possible influence on the transport processes. © 1984 Academia, Publishing House of the Czechoslovak Academy of Sciences.