In vivo studies on reversal of chloroquine (CQ) resistance by cimetidine (CIM) were carried out in thirty five patients infected with Plasmodium falciparum in Omdurman Hospital for Tropical Diseases and Elhaj Yousif area, Khartoum Sudan. Parasites were considered resistant if still present in peripheral blood circulation, three days after the start of standard dose of CQ treatment. Patients with CQ resistant parasites were admitted to the hospital and given CIM (800 mg in two divided doses), 48 hours after the start of standard dose of chloroquine treatment. They were followed clinically and microscopically, daily for one week and then discharged. Treatment in the dose used was found to reverse CQ resistance in (70%) of the patients studied within three days of CIM treatment. No side effects were reported by any of the patients. Glutamicoxaloacetic transaminase (GOT) content in the treated group was found to be 31.57 I.U/L (± 0.85), protein content 6.5 g/dl (± 0.85) and the uric acid 6.5 mg/dl (± 0.71). The values of GOT, protein and uric acid in the resistant group were found to be slightly higher than those in by the sensitive one. Key words: chloroquine, cimetidine, resistance, reversal