There is clinical and experimental evidence that lack of thyroid hormones may affect the composition and structure of the interstitium. This can influence the relationship between volume and pressure during changes in hydration. Hypothyrosis was induced in rats by thyroidectomy 8 wk before the experiments. Overhydration was induced by infusion of acetated Ringer, 5, 10, and 20% of the body weight, while fluid was withdrawn by peritoneal dialysis with hypertonic glucose. Interstitial fluid pressure (P_i) in euvolemia (euvolemic control situation) and experimental situation was measured with micropipettes connected to a servocontrolled counterpressure system. The corresponding interstitial fluid volume (V_i) was found as the difference between extracellular fluid volume measured as the distribution volume of ⁵¹Cr-labeled EDTA and plasma volume measured using ¹²⁵I-labeled human serum albumin. In euvolemia, V_i was similar or lower in the skin and higher in skeletal muscle of hypothyroid than in euthyroid control rats, whereas the corresponding P_i was higher in all tissues. During overhydration, P_i rose to the same absolute level in both types of rats, whereas during peritoneal dialysis there was a linear relationship between volume and pressure in all tissues and types of rats. Interstitial compliance (C_i), calculated as the inverse value of the slope of the curve relating changes in volume and pressure in dehydration, did not differ significantly in the hindlimb skin of hypothyroid and euthyroid rats. However, in skeletal muscle, C_i was 1.3 and 2.0 ml · 100 g¹ · mmHg¹ in hypothyroid and euthyroid rats (P < 0.01), with corresponding numbers for the back skin of 2.7 and 5.0 ml · 100 g¹ · mmHg¹ (P < 0.01). These experiments suggest that lack of thyroid hormones in rats changes the interstitial matrix, again leading to reduced C_i and reduced ability to mobilize fluid from the interstitium.