The influence of TNF alpha on tumour growth rate has been attributed to its effects on the vascular bed and blood flow. The aim of our study was to investigate the effects of pharmacological doses of TNF alpha on the tumour vascular bed and to quantify blood flow in an experimental hepatoma during a more extended period after TNF-alpha exposure than hitherto reported. In Lister rats, a syngeneic rat hepatoma was implanted on the dorsum of the right hind foot. TNF alpha was given i.v. The injection was repeated after 24 hr. Tumour blood flow was estimated before and 1, 24, and 96 hr after TNF-alpha administration with the 133Xe-washout technique. The passage of microspheres through the tumour vascular bed (non-entrapment), as a measure of vascular occlusion, was estimated 4 and 96 hr after TNF-alpha administration. Tumour growth rate was measured. The tumours were subjected to histological examination and the sensitivity to TNF alpha in vitro was tested. A reduction of tumour blood flow was observed in TNF-alpha-treated groups. Tumour growth rate was equally increased after 96 hr in both the TNF-alpha groups as compared with controls. There was no significant change in non-entrapment for the TNF-alpha-treated rats as compared with controls. Histology revealed extensive necrosis and thrombosis in tumours. TNF alpha had no effect on the viability of the cloned hepatoma cell line in vitro.