The pattern of RNA synthesis during male gametogenesis of Brassica napus was studied using 3H-uridine autoradiography. No incorporation of isotope occurred in the newly released microspore and the nonvacuolate, furrowed microspore. Peak incorporation of label during male gametogenesis occurred in the uninucleate, furrowed microspores showing various degrees of vacuolation. In this microspore stage, silver grains were localized in the nucleus and cytoplasm. Moderate incorporation of the isotope occurred in the nulceus of the vacuolated microspore. After the microspore mitosis, isotope incorporation occurred predominantly in the nucleus of the vegetative cell with little or no incorporation in the generative cell. In tricellular pollen, no incorporation of isotope was observed in both the vegetative nucleus and the sperms. Silver grains almost completely disappeared from tricellular mature pollen grains ready to germinate.