Effects of batatasins I, III and V, phenolic growth inhibitors occuring in d o r m a n t bulbils of Dio- scorea batatas Decne., on photosynthetic reactions of chloroplasts from spinach (Spinacia oleracea L.) and on respiration o f m i t o c h o n d r i a from potatoes ( S o l a n u m tuberosum L.) were investigated. In chloroplasts, the batatasins effectively inhibited CO2-dependent oxygen evolution and electron flow from water to acceptors such as d i c h l o r o p h e n o l i n d o p h e n o l , ferricyanide and methylviologen. P h o t o s y s t e m -I dependent electron t r a n s p o r t from ascorbate to oxygen was stimulated. The p r o t o n conductivity of thylakoid m e m b r a n e s was increased and p h o s p h o r y l a t i o n was uncoupled from electron transport. Inhibition of electron t r a n s p o r t with water as electron d o n o r a p p e a r e d to precede uncoupling. In mitochondrial, batatasin I did not much inhibit succinate-dependent O2 u p t a k e in the absence of A D P , but caused strong inhibition in the presence of A D P . Batatasins III and V inhibited oxygen u p t a k e irrespective of the presence or absence of A D P . Inhibition of chloroplast and m i t o c h o n d r i a l reactions by batatasins was shown to be reversible.