Rats of the inbred Buffalo strain have previously been reported to be susceptible to thyroiditis, as defined by histology. We have studied the endocrinology of the pituitary-thyroid axis of this strain by making direct measurements of the plasma concentrations of TSH and T4 in untreated, adult Buffalo rats of both sexes. Plasma PRL levels were also measured. All hormone determinations were by RIA. In addition, relative thyroid weights were noted and, in many cases, preliminary assessment of thyroid histology was made. Our principle findings were as follows. 1) Decreased thyroid function, in addition to the previously reported histological abnormalities, was found to occur spontaneously among the rats studied. Indications of decreased function included elevations of plasma TSH and thyroid weight and depressions of plasma T4. We estimated the incidence of unequivocal thyroid disease as approximately 3% in each sex. 2) Basal plasma PRL concentrations of Buffalo rats averaged three to four times higher than those of outbred CD rats. Our findings strongly suggest that rats of the Buffalo strain will provide a good model for the study of thyroid failure of varying degrees and concomitant changes in the circulating levels of pituitary hormones.