Pisumseeds were soaked in water and in Cytochalasin B (CB) for 24 and 48 h, then left in the dark to germinate for 5 days. During this first phase of germination the ultrastructure of the cotyledonary cells was examined, seed respiration was measured, the contents of storage substances and acid phosphatase activity in the cotyledons and axes were separately evaluated. The ultrastructure of cotyledonary cells treated with CB appeared to be normal, thus indicating that the imbibition process was unaffected by CB. Respiration showed a similar pattern in all tested samples although its intensity was considerably lower in treated seeds. Treated axes and cotyledons, however, had a higher starch, soluble sugar and protein content but a lower acid phosphatase activity as compared to controls. These results suggest that the effects of CB on the rate of germination and cell wall extension, as reported in previous studies, may be due to an inhibitory effect of CB on mobilization and utilization of reserve substances.