To determine if the pharmacokinetics of high doses of zidovudine (AZT) (that is, how fast AZT reaches the blood, what concentration of AZT is attained in the blood, and how long AZT remains in the blood) changes from day to day in the same patient. Also to determine whether the pharmacokinetics of AZT is changed when trimethoprim/sulfamethoxazole (SMX/TMP) is given at the same time, or whether the pharmacokinetics of SMX/TMP is altered by AZT given at the same time.