Bacillus thuringiensis (Bt) transgenic (KMD1) and non-transgenic (KMD1's parental variety Xiushui 11) rice flours were assessed in a 90 day feeding test with rats. KMD1 contained a synthetic cry1Ab gene from Bt, and selection marker genes nptII and hpt linked in tandem. In the .ltoreq.64g kg-1 body wt. (BW) dosage range (Bt transgenic rice flour composed 64% of the ingredients of the diet), no adverse effects of Bt rice on rats were obsd. in terms of animal behavior, wt. gain and feed utilization rate. Necropsy at the end of the expt. indicated that neither pathol. lesions nor histopathol. abnormalities were present in organs such as liver, kidneys, intestines and testes of rats in both test and control groups by macroscopic and microscopic pathol. In addn., no significant differences (P>0.05) were obsd. in relative organ wts., haemograms and blood indexes of rats between test and control groups. Several serum parameters of female rats were found to be significantly different between Bt and non-Bt diets, but the values of these parameters were still within the normal ranges of values for rats of this age and sex. These results demonstrated that Bt rice flour at a dosage of 64g kg-1 BW, Bt toxin and NPTII and HPT proteins have no toxic effects on rats.