This paper describes the study of the pharmacodynamics of two 11C-labelled nitrosoureas, 1,3-bis-(2-chloroethyl) nitrosourea (BNCU) and sarcosinamide chloroethylnitrosourea (SarCNU), both labelled in the carbonyl position. Distribution of the radioactivity as measured by positron emission tomography was compared to the distribution of radioactivity observed after injection of 68Ga-EDTA, this being used as an indicator of the blood-brain barrier integrity around the brain tumor. Data suggest that the new nitrosourea, SarCNU, most likely enters brain tissue by different mechanism(s) than BCNU, which enters by diffusion. Data also indicate that use of SarCNU may result in a better tumor to brain ratio than BCNU.