Tamoxifen and its principle metabolite N-desmethyltamoxifen can modulate multi-drug resistance in vitro. Tamoxifen 480 mg/day was given for 6 days with oral etoposide on days 4-6 to 17 patients with advanced solid tumours. Venous thrombosis (2 patients), reversible neurological toxicity (1 patient), and WHO grade III nausea/vomiting (3 patients) related to tamoxifen were observed but overall toxicity was manageable. One partial response occurred in 15 assessable patients. Mean plasma concentrations of tamoxifen and N-desmethyltamoxifen increased to 4.3 μmol/l and 2.7 μmol/l, respectively, by day 6. Plasma concentrations corresponding to active in vitro levels were attained by most patient