Thymidylate synthase (TS), an enzyme which acts by utilizing 5,10-CH(2)FH(4) in the reductive methylation of deoxyuridylate (dUMP), is required for the de novo synthesis of thymidylate and is a potential chemotherapeutic target. ZD1694 is a new quinazolone folate analog that directly inhibits TS. This phase I trial and pharmacokinetic study will describe and define the toxicities, determine the MTD, and describe the plasma pharmacokinetics of ZD1694 in pediatric patients with refractory cancer. The starting dose for this trial will be 2.0 mg/m(2) administered as a 15 minute IV infusion every 21 days.