The possibility of parathyroid dysfunction as a casusal factor in the osteopetrosis of the tl rat was explored by evaluating serum C and immunoreactive parathyroid hormone (iPTH) concentrations in this strain of rats, as compared with those of phenotypically normal littermate (LM) and non-littermate (NLM) control groups. The mean serum Ca concentration in the tl rat was not significantly different from that of the NLM group, although it was less than that of the LM group. All Ca values were within the normal range. The mean serum iPTH concentration in the tl rats was not significantly different from those of either control group. The adult tl rat probably has adequate but not excessive PTH secretion. Osteopetrosis and its manifestations in the adult tl rat are not caused by an abnormality of parathyroid function.