Wolffia arrhiza (L.) Wimm. plants precultivated under different radiation conditions were exposed to UV-B at low or strong "white" irradiance. Photosynthetíc electron transport (Hill actívity) was more damaged by UV-B under 6 W m"^ than at 15 W m‘2 supplemental "white light" (WR). The inhibitory effect was more pronounced using red radiation (RR) precultivated plants compared to precultívatíon at blue radiation (BR) or WR. Hill reaction in RR grown W. arrhiza with a high growth rate was more affected by UV-B - in comiection with DCMU and 1,5- diphenyl carbazide - than of plants with a low growth rate. No effect was observed using plants precultivated at BR or WR.