The primary purpose of this Phase I protocol is to develop an orally administered regimen of fluorouracil (5-FU) given with fixed doses of leucovorin (LV) and 776C85 (GW776), a mechanism-based inhibitor of dihydropyrimidine dehydrogenase (DPD), the rate-limiting enzyme involved in the catabolism of 5-FU. In the presence of 776C85, 5-FU is cleared by renal mechanisms. The schedule employed is intended to mimic the pharmacologic profile associated with a 24 hour weekly continuous infusion of 5-FU without the need for an indwelling central venous catheter. The target population is adult cancer patients with solid tumors.