Thirty-one patients were entered into a pilot study combining oral quinidine with epirubicin 100 mg m-2 as first line chemotherapy in advanced breast cancer. Three patients were treated with quinidine 1 g b.d., and developed symptoms of toxicity. Of eight subsequent patients treated with quinidine 500 mg b.d., two experienced tiredness and nausea and one severe oral toxicity with epirubicin. The remaining 20 patients received quinidine 250 mg b.d.; one developed cinchonism and one malaise, the remainder showing no excess toxicity compared with epirubicin alone. The median nadir WBC was similar with or without quinidine (2.3 vs 1.6 x 10(9) l-1) as was median nadir platelet count (175 vs 157 x 10(9) l-1). There was no evidence of significant cardiac toxicity. The median plasma quinidine level achieved was 5.6 mumol l-1 (range 2.1-22.1), which is within the range of concentrations which is effective in vitro at reversing experimental anthracycline resistance. A randomised controlled study is proposed to assess the impact of this potential modulation on the efficacy of epirubicin in advanced breast cancer.