Capillary permeability of 99mtechnetium-diethylenetriaminepenta-acetic acid (99mTc-DTPA; MW 485.0) and 51chromium-ethylenediaminetetra-acetate (51Cr-EDTA; MW 340.2) was studied in an in vivo canine heart preparation by the single injection, residue detection (SIRD) method. In experiments on open chest dogs (group A) these indicators were administered separately as bolus injections into a cannulated diagonal branch of the left anterior descending coronary artery (LAD) and the curve of the response function was recorded by external activity registration. In further experiments on closed and open chest dogs (group B) 99mTc-DTPA was injected via a coronary angiography catheter into the ostium of the left coronary artery and the response curve was obtained by external registration. Regional myocardial perfusion coefficients were determined by the local 133xenon washout technique and from kinetic analysis of SIRD experiments. Plasma perfusion coefficients calculated by kinetic analysis of response curves from 99mTc-DTPA SIRD experiments (group A) were on average 98 v 102 ml.min-1.100 g-1 in 51Cr-EDTA experiments, in good agreement with the average plasma flow rate of 92 ml.min-1.100 g-1 determined by local 133Xe washout technique. Mean values of the extraction fractions (E) were 0.463 and 0.475 for 99mTc-DTPA and 51Cr-EDTA, and the permeability-surface area products (PdS) were 52.8 and 57.5 ml.min-1.100 g-1 respectively. At the conventional capillary surface area estimate of 500 cm2.g-1 the permeability coefficient (Pd) for 99mTc-DTPA was 1.76.10(-5) cm.s-1 and for 51Cr-EDTA 1.92.10(-5) cm.s-1.(ABSTRACT TRUNCATED AT 250 WORDS)