Species isoforms of histamine H 2 -, H 3 -, and H 4 -receptors differ in their pharmacological properties. The study aim was to dissect differences between the human H 1 R (hH 1 R) and guinea pig H 1 R (ghH 1 R). We coexpressed hH 1 R and gpH 1 R with regulators of G-protein signaling in Sf9 insect cells and analyzed the GTPase activity of G q -proteins. Small H 1 R agonists showed similar effects at hH 1 R and gpH 1 R, whereas bulkier 2-phenylhistamines and histaprodifens were up to Ïł10-fold more potent at gpH 1 R than at hH 1 R. Most 2-phenylhistamines and histaprodifens were more efficacious at gpH 1 R than at hH 1 R. Several first-generation H 1 R antagonists were Ïł2-fold, and arpromidine-type H 1 R antagonists up to Ïł10-fold more potent at gpH 1 R than at hH 1 R. [ The Phe-1533 Leu-153/Ile-4333 Val-433 double mutant expressed excellently but only partially changed the pharmacological properties of hH 1 R. Small H 1 R agonists and 2-phenylhistamines interacted differentially with human and guinea pig H 2 R in terms of potency and efficacy, respectively. Our data show the following: 1) there are differences in agonist-and antagonistpharmacology of hH 1 R and gpH 1 R encompassing diverse classes of bulky ligands. These differences may be explained by higher conformational flexibility of gpH 1 R relative to hH 1 R; 2) Phe-153 and Ile-433 are critical for proper folding and expression of hH 1 R; and 3) H 2 R species isoforms distinguish between H 1 R agonists.Histamine serves as a neurotransmitter and autacoid and acts through specific H x Rs designated as H 1 R, H 2 R, H 3 R, and H 4 R, respectively (Hill et al., 1997;Hough, 2001). The H 1 R couples to G q -proteins. Numerous H 1 R agonists and antagonists are known. H 1 R agonists are divided into three classes ( Fig. 1): 1) small agonists (2-4) derived from histamine (1), 2) histamine derivatives with bulkier aromatic substituents at position 2 of the imidazole ring (5-18), and 3) histaprodifens, e.g., compounds 19 to 23 Zingel et al., 1995;Elz et al., 2000). H 1 R agonists are important experimental tools to analyze H 1 R function in cellular and organ systems (Zingel et al., 1995;Hill et al., 1997). H 1 R antagonists are commonly divided into sedating (first-generation, 24-32) and nonsedating (second-generation, 41-45) antagonists (Fig. 2). Today, especially the second-generation H 1 R antagonists are of great importance for the treatment of allergic diseases (Hill et al., 1997). Guanidines 33, 34, and 36 to 39 derived from arpromidine (35) are dual H 2 R agonists/ H 1 R antagonists (Buschauer, 1989).The availability of H x R cDNAs allowed for the comparison of the pharmacological properties of H x R species isoforms in recombinant systems under identical experimental conditions. Such expression studies uncovered species differences This work was supported by the National Institutes of Health COBRE Award 1 P20 RR15563 and matching support from the State of Kansas and the University of Kansas (R.S.), a grant from the Army Research Office (DAAD 19-00-...