Thirteen dogs were entered into a pilot study to assess the toxicities associated with polyamine biosynthetic enzyme inhibitors as heat sensitizing drugs and whole body hyperthermia either alone or in combination. Disease-free and tumour-bearing animals were entered in an effort to assess the response of canine lymphoproliferative disorders as well. Conclusions reached were as follows. (1) A large tumour burden precludes treatment. (2) Liver involvement and decreased platelet numbers would appear to offer a grave prognosis for survival and should be assessed closely in determining eligibility. (3) Polyamine biosynthetic enzyme inhibitors and whole body hyperthermia appear to be tumoricidal alone or in combination, although less effective and with greater toxicities than accepted chemotherapeutic regimens. (4) Heat sensitization in vivo has not yet been demonstrated.