1. 1. Since dimethyl sulfoxide (DMSO) is a solvent which is often used for drugs in animal studies, we investigated the effect of a daily administration of DMSO on the telemetrically obtained electrocardiogram (ECG) in freely moving male Balb/c mice. 2. 2. During treatment with 4.5 ml 100% DMSO/kg i.p. 5 days per week during 3 weeks, DMSO caused substantial cardiotoxicity. The ST-interval increased significantly after 1 week by 2.2 ± 1.3 msec and also the ECG wave form changed completely in time. 3. 3. During treatment with 4.5 ml 50% DMSO/kg i.p. 5 days per week during 3 weeks, no significant difference was observed compared with the control animals. 4. 4. During the entire study the maximal heart rate and body weight remained constant in all treated groups. 5. 5. The data indicate that DMSO can not be used in a 100% concentration to dissolve compounds that are tested for protection against the cardiotoxicity of cytostatics. © 1995.