Protein kinase activity in the embryos of rice (Oryza sativa L. cv. Nipponbare) during seed soaking in water was assessed under various conditions, The histone III-S phosphorylating activity in the membrane fraction prepared from control seedlings treated with water was increased by the addition of Ca2+ (0.2 mM), and it increased further up on the simultaneous addition of phosphatidylserine (PS), By contrast, the activity in extracts prepared from ABA-treated seeds increased by Ca2+ alone and PS did not augment this increased level of activity, In vitro phosphorylation of the membrane fraction from control seeds in the presence of Ca2+ and PS resulted in phosphorylation of two proteins with molecular masses and isoelectric points of 40 kDa/8.9 and 40 kDa/7.6, respectively, When seeds were treated with exogenous ABA, phosphorylation in vitro of these two proteins was dependent only on the addition of Ca2+, N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) did not block phosphorylation of these proteins, which is dependent on Ca2+, The varieties and quantities of phospholipids in the seeds after soaking in water containing ABA were the same as those after soaking only in water, Phosphorylation of the 40-kDa proteins in vitro in the membrane fraction from control seeds required higher Ca2+ concentrations (1-2 mM) compared to that observed in the membrane fraction from ABA-treated seeds.