TO investigate the possible role of 5HT1Band/or 5HT1Dreceptors in controlling neurogenic inflammation, we performed a co-localization study of the mRNA for 5HT1Band 5HT1Dreceptors and of substance P or calcitonin gene-related peptide (CGRP) mRNA in the guinea pig trigeminal ganglion using double labellingin situhybridization techniques. The 5HT1Dreceptor mRNA is abundant whereas 5HT1Breceptor mRNA is scarce. The vast majority of cells containing substance P mRNA also contained 5HT1Breceptor mRNA, but very few cells expressed substance P mRNA and 5HT1Dreceptor mRNA. Both receptor mRNAs were co-localized with CGRP mRNA. Hence, 5HT1Dreceptors may control the release of CGRP only, whereas 5HT1Breceptors may control the release of both substance P and CGRP. The question remains whether selective 5HT1Dagonists will have migraine abortive properties.