The rates of incoporation in vivo of 131Iododeoxyuridine (mIUdR) into the tumour and into 10 organs of tumour-bearing rats was investigated after partial synchronization with hydroxyurea (HU). The rate of DNA synthesis was reduced to 5% of the normal value over aperiod of 8.5 h by six successive injections of HU (50 mg/kg body weight). Commencing 1.5 h after the last HU injection, groups of 6 animals were injected with 131IUdR at intervals of 0.5 h. The animals were killed 48 h after this treatment and 131IUdR activity in the individual organs and in the tumour was determined. The results showed that the maximal uptake of 131IUdR after partial synchronization varied from organ to organ (not increased: brain, blood; increased by a factor of 1.5-1.8 duodenum, liver, spleen; increased by a factor of 2.1-2.5 lung, tumour, muscle, bone; increased by a factor of 3.6-4.5 stomach, kidney). Moreover, the time to reach maximal incorporation after HU treatment is different according to the tissue under investigation. The use of 131IUdR as a tumour-localizing agent in scintigraphy of tumour-bearing rats previously injected with HU showed that this method of partial synchronization clearly facilitates the detection of tumours in the lung area.