The neurotoxin DSP-4 has, in this laboratory, previously been shown to upregulate cholecystokinin (CCK) binding in the frontal cortex and the hippocampus in rats. The present study investigated the effect of DSP-4 on CCK peptide levels and CCK receptor binding in some rat brain areas. Sprague-Dawley rats were given single injections of DSP-4 (10 and 50 mg/kg) and examined after 1 week. Wistar rats were given an injection of DSP-4 (50 mg/kg) and evaluated 1 week and 4 months after treatment. No significant changes in CCK levels or in CCK binding were found. It has been found that stress produced by handling of the rats immediately before decapitation can result in an increase in cortical CCK receptor binding. This finding led to the speculation that the upregulation of CCK binding after DSP-4 treatment which was reported earlier, reflects a stress-potentiating effect of DSP-4.