The relative effectiveness of Ca-DTPA and Zn-DTPA as a function of the dose, number of doses and time of their administration in removing 234Th that had been administered as the citrate or nitrate from various rat organs was investigated. With a single early treatment (1.5 min after intravenous injection of 234Th), Zn-DTPA was clearly less effective than Ca-DTPA over the whole dose range (30 mumol/kg body weight-1000 mumol/kg body weight) tested. In the skeleton, 1000 mumol Zn-DTPA/kg removed as much 234Th (about 50%) as 30 mumol Ca DTPA/kg. Prompt treatment with 100 mumol Ca-DTPA/kg body weight diminished the skeletal 234Th content by about 70%; but when it was delayed by 6 h or 4 d, the 234Th content was reduced by only about 20% and 10%, respectively. The effect of DTPA in the bone was independent of 234Th compound, while in the soft tissues 234Th injected as nitrate was less affected than the citrate. The effect of DTPA was potentiated when treatment was repeated; the end effect was dependent on the time of the first DTPA injection and on the total number of doses. Thus, about 90% and 50% less 234Th was found in the bones after repeated treatment started at 1.5 min and 4 d after administration of 234Th, respectively. Under the conditions of a delayed repeated treatment, Ca-DTPA and Zn-DTPA were equally effective.