The sequence of chemotherapy administered prior to cranial irradiation rather than the more traditional order of radiation followed by chemotherapy is currently being evaluated. This rodent study was designed to assess the sequencing of radiation therapy and chemotherapy administered with osmotic blood-brain barrier disruption (BBBD). Drug delivery and acute toxicity were evaluated. Two clinically relevant chemotherapy regimens were given with BBBD: intraarterial methotrexate (MTX, 1 g/m2), or a combination of intraarterial carboplatin (200 mg/m2) and i.v. etoposide (200 mg/m2). In a randomized protocol, the standard rodent model of 2000 cGy as a single fraction using parallel opposed portals was administered 30 days prior to, concurrent with (24 h prior), or 30 days after these two chemotherapeutic regimens. A total of 72 animals was evaluated in this study. The administration of external beam radiation therapy either prior to or concurrent with the administration of a high molecular weight marker 14C-labeled dextran 70 (Mr 70,000), or a low molecular weight marker 3H-labeled MTX (Mr 456) resulted in a statistically significant (P < 0.01) decrease in drug delivery when compared to animals not receiving cranial irradiation. Seizures were observed in 26% of the animals that received radiation prior to the administration of intraarterial MTX after BBBD. It did not matter whether the radiotherapy was administered 30 days prior to or concurrent with MTX. Seizures were not seen in any other group. The mortality in animals receiving radiotherapy 30 days prior to chemotherapy was significantly (P = 0.03) higher than the mortality in control animals receiving chemotherapy after osmotic BBBD, but no radiation. Drug delivery was significantly decreased when the animals received prior radiotherapy; the administration of radiation prior to MTX with BBBD resulted in an increased incidence of seizures, and there was a significant increase in mortality when cranial irradiation was given 30 days prior to chemotherapy administered with BBBD. With regard to delivery and toxicity, chemotherapy with BBBD administered prior to radiotherapy may have advantages over the other sequences utilizing chemotherapy and cranial irradiation.