Positron emission tomography is a potential method for exploring the biochemical behaviour of tumours. In 28 patients with known neoplastic lesions a comparison was made between two agents which are known to be accumulated in malignant tumours, viz. 13N-ammonia and 11C-aminocyclopentane carboxylic acid (ACPC). Absolute concentration of both agents in various tumoural tissues and normal organs was calculated. As a rule a parallelism was found between the two tracers as to their accumulation in a given tumour, although the concentration was often higher for ACPC. In normal tissues the ACPC accumulation was either lower or at most equal to NH3 levels. As tumour tracer ACPC is superior to NH3 because of its higher absolute accumulation in many neoplastic lesions and its lower uptake in various non-tumorous tissues. ACPC concentration in tumours seems to be largely independent of blood flow.