Both 125I-labeled bovine [b] TSH [thyrotropin] and rat[r]TSH bind rapidly and saturably to a particulate fraction of thyroid glands of rats. Unlabeled bTSH and rTSH displaced 125I-labeled rTSH from binding sites in the thyroid particulate in an analogous manner. Specific binding of tracer concentrations of [125I]bTSH to the rat thyroid particulate fraction was several times higher than that of [125I]rTSH. Rat GH [somatotropin] failed to displace 125I-labeled bTSH, rat FSH [follitropin] had only a slight inhibitory effect at the concentrations used, but rat LH [lutropin] displaced labeled bTSH better than bTSH did. Specific binding of [125I]bTSH to thyroid (21.6%) was several times greater than that to spleen (5.6%) liver (3.6%) or kidney (2.4%). At 37.degree. C, maximum binding was achieved within 30 min but quickly decreased to lower levels. Incubation at temperatures above 4.degree. C for 3 h resulted in a reduction in binding. At 4.degree. C, binding of [125I]bTSH to the particulate fraction was rapid, saturable and remained stable for 4.5 h but decreased within 21 h. Optimum binding occurred at pH 6.0 but experiments were conducted at pH 7.45. An excellent correlation coefficient [r] (0.994) was apparent between maximum binding capacity, as determined from analyses of Scatchard plots, and particulate protein concentration. Scatchard analysis of bTSH binding was usually most consistent with the presence of a single binding site, although in some experiments a lower affinity, higher capacity site also seemed to be present. The mean association constant of the single binding site when 1 was found or the high affinity low capacity site when 2 sites were evident was 2.2 .+-. 0.5 .times. 108 M-1 (mean .+-. SD) in 4 experiments. In the same experiments, the maximum binding capacity averaged 117.5 .+-. 43.0 .mu.U[units]/.mu.g, 2.9 .+-. 1.1 ng/.mu.g, or 6.3 .+-. 2.3 .times. 1010 sites/.mu.g particulate protein. The system here described, which makes use of a particulate fraction from the rat thyroid, may prove a valuable model for studying the role of alterations in TSH receptors in the response of the thyroid to a variety of stimuli.