Bradykinin receptors have been subdivided into at least two major pharmacological subtypes, B1 and B2. The cDNAs encoding functional B2 receptors have recently been cloned, but no molecular information exists at present on the B1 receptor. In this article, we describe experiments examining the possible relationship between the mRNAs encoding the B1 and B2 types of receptor. We showed previously that the human fibroblast cell line WI38 expresses both B1 and B2 receptors. In this report, we describe oocyte expression experiments showing that the B1 receptor in WI38 human fibroblast cells is encoded by a distinct mRNA approximately 2 kb shorter than that encoding the B2 receptor. We have used an antisense approach in conjunction with the oocyte expression system to demonstrate that the two messages differ in sequence at several locations throughout the length of the B2 sequence. Taken together with the mixed pharmacology exhibited in some expression systems by the cloned mouse receptor, the data indicate that B1-type pharmacology may arise from two independent molecular mechanisms.