Transpiration of arable crops is one of the basic components of their water balance. The sap flow in Brassica napus L. plants (Q; kg h-1) was measured in fields in central Bohemia during the vegetation period in 2005. The selected measuring interval was that from the pod formation stage to the maturation stage. Q was observed using a sap flow meter T 4.2 (EMS, Brno). The decrease in actual Q values after BBCH 84 phase accession was documented by means of the calculated Qcalc values, which were estimated from the photosynthetic active radiation and the vapor pressure deficit values. The average daily Q value of a single B. napus plant achieved 0.33 kg day–1 within the BBCH 78–83 phase. Within the BBCH 84–89 phase, the average daily Q value was only 0.09 kg day–1. The average water consumption of the whole plant cover was 2.8 mm for 51 plants per m2.