Acute hypoxia is often induced in rodent tumors during studies of the oxygenation or the therapeutic responses of the tumors, either by clamping the blood supply to the tumors or by asphyxiating the hosts with nitrogen. Analyses of data from such experiments generally assume that these processes have no effects other than the induction of hypoxia and that uniform, severe hypoxia is produced throughout the tumors. The studies described in this report test several aspects of these assumptions using BA1112 rat rhabdomyosarcomas and EMT6 mouse mammary tumors. Both clamping and asphyxiation appear to be effective in producing hypoxia in the tumors. Induction of artificial hypoxia for the times necessary for irradiation was not toxic to the tumor cells and generally did not alter the growth of unirradiated tumors. However, the techniques are not without significant effects. Prolonged clamping produced extensive cytotoxicity. Clamping BA1112 tumors for 30 min and removing the clamp just before irradiation altered the tumor cell survival curve and the TCD50. Furthermore, anesthesia and/or restraint (necessary during clamping) have significant effects on tumors and hosts. The data show that the assumptions underlying the use of clamping and N2-asphyxiation to produce hypoxia for short periods in vivo are generally reasonable for BA1112 and EMT6 tumors, but illustrate the necessity for carefully examining the effects and efficacies of the procedures in each tumor/host system.