This study aimed to investigate the long-term toxicity of a preventive regimen of tamoxifen (TAM) and recombinant human interferon alpha 2b (rHuIFN alpha 2b) on the uterine responsiveness of tumour-bearing rats. The experimental tumour was induced by dimethylbenz(a)anthracene (DMBA) in virgin female albino rats and the therapy was started two months after carcinogen administration. The acute effect of DMBA on the uterine sensitivity was also assessed 24 h post-carcinogen. The uterotonic potentials of oxytocin and prostaglandin F2 alpha (PGF2 alpha) were markedly reduced in the control tumour-bearing group as compared to the normal one. Similarly, acute DMBA administration showed reduced uterine sensitivity to both agents. Treatment with either TAM or combined TAM/rHuIFN alpha 2b did not affect the uterine response to either oxytocin or PGF2 alpha, while rHuIFN alpha 2b increased the uterine sensitivity to oxytocin but not to PGF2 alpha. These data indicate that the carcinogenic agent per se and the presence of tumour reduce the contractile response in the rat uterus to oxytocic agents. Moreover, combined TAM/rHuIFN alpha 2b does not markedly affect the uterine sensitivity in DMBA-induced mammary carcinoma-bearing rats.