Somatic embryos or shoots can be induced in vitro on immature zygotic embryos of sunflower (Helianthus annuus L.) depending on the sucrose concentration of the culture medium. On a medium containing 3% sucrose only shoots are induced while on a medium containing 12% sucrose somatic embryos are formed. Although these two different reactions are not induced by changes in the concentration of externally supplied growth regulators, the induction of both morphogenic events is probably depending on internal hormone regulation, in response to the medium composition. We have studied the effect of the external supply of some growth regulators known to induce organogenesis, such as cytokinins, or somatic embryogenesis, such as auxins and abscisic acid (ABA). We have also studied the effect of auxin polar transport inhibitors on the developmental response. Addition of auxins or abscisic acid to the organogenic medium (3% sucrose) induced somatic embryogenesis whereas addition of auxin polar transport inhibitors led to shoot formation under otherwise embryogenic conditions (12% sucrose). Moreover, among the cytokinins tested, zeatin had an unexpected embryogenic effect independently of the sugar concentration of the medium. Possible relevance of the different hormones, for the morphogenic response and their interactions, is discussed.