This report describes a comparative study between technetium-99m ethyl cysteinate dimer (ECD) and 99mTc-hexamethylpropylene amine oxime (HMPAO) in five neurological patients. The conversion kinetics of the tracers in the blood from forms capable of diffusion across the blood-brain barrier to non-diffusible forms were studied by arterial sampling and rapid octanol extraction. We observed that HMPAO has a faster conversion rate in the blood but that the fraction of the injected dose available for brain extraction is higher than in the case of ECD. Regional brain concentrations of the tracers were measured with single-photon emission tomography (SPET) 35 min and 60 min after the injection and remained stable within this interval. On the basis of the measurements of the arterial input and of SPET brain concentrations of the tracers, the regional steady-state influx constants (Ki in ml/min/g) were determined for several brain regions. In the grey matter the Ki values were (mean +/- SD) 0.32 +/- 0.03 and 0.35 +/- 0.04 for HMPAO and ECD, respectively; in the white matter the values were 0.23 +/- 0.01 and 0.23 +/- 0.02, respectively. The Ki values of the two tracers in corresponding regions were closely correlated (P < 0.001). The correspondence of the Ki values of ECD and HMPAO demonstrates that ECD can also be considered a tracer that may be used for quantitative measurements of brain perfusion.