A statistically significant circadian rhythm in tolerance of 226 male B6D2F1 mice synchronized with LD 12:12 for 4'-O-tetrahydropyranyl-adriamycin (THP) was demonstrated. Four intravenous dosages (18, 25, 32 and 40 mg/kg) and six different dosing times (3, 7, 10, 14, 19 and 23 hr after light onset-HALO) were compared. Survival rate, body weight loss and leukopenia depended on both the dose and time of injection. The overall survival rate varied between 83% (light-rest span) and 56% (dark-activity span) (chi2 = 17; d.f. = 2; P less than 0.001). Maximal body weight loss occurred 4-5 days after drug injection. Total leukocyte counts were determined on these days. Both body weight loss and leukopenia were reduced by approximately 100% in those mice injected in their late rest span (7-10 HALO) as compared to those treated in the middle of their activity span (19 HALO). Circadian rhythms in day-60 survival rate, body weight loss and leukopenia were statistically validated by cosinor analysis, with estimated peak times (acrophases) occurring respectively at 7:30, 9:20 and 8:40 HALO. Minor cardiac lesions consisting of diffuse vacuolization and loss of muscular striation were observed in histologic sections from 3/32 hearts (16 controls, 16 treated). All three corresponded to THP given at 19 (2/2 mice) or 23 HALO (1/4 mice). Thus lethal, hematologic and possibly cardiac tolerance for THP were largely optimized by administering the drug to mice in their late span (7-10 HALO).