Chronic atropine treatment (14 days, 20 mg X day-1 X kg-1 SC) caused a 75% increase in the number of VIP receptors in the rat cerebral cortex. The affinity of these receptors for 125I-VIP was not altered significantly by the atropine treatment. The same treatment led to a 20% decrease in VIP tissue levels. Muscarinic receptor number was also increased by 26%. The results indicate that interactions between VIP- and muscarinic receptors may be of importance in the rat cerebral cortex.