A group of 32 patients with perennial allergic rhinitis sensitized exclusively to house dust and mites were evaluated for "in vivo" modifications (skin tests for histamine, house dust and mites) and "in vitro" alterations (seric IgE, specific IgE, histamine release curve to different concentrations of Dermatophagoides pteronyssinus and histamine release curve to this pneumoallergen after incubation with H1 receptor antagonists) to four antihistamines (astemizole, azatadine, mequitazine and terfenadine). The patients were divided into 4 groups of 8 and the "in vivo" and "in vitro" tests were performed on days -1, +15 and +30 of administration of these drugs. The reduction in the size of the wheal (prick test) for histamine when comparing days -1 and +15 and -1 and +30 was 69% and 79% (astemizole), 64% and 71% (azatadine), 69% and 64% (mequitazine) and 61% and 64% (terfenadine), respectively. The difference in the mean size of the wheal in mm2 for house dust, Dermatophagoides pteronyssinus and farinae was statistically significant (p less than 0.001) except when comparing the results obtained for mequitazine on days -1 and +15. This was also the case on days -1 and +30 for terfenadine. The values of the serum IgE and specific IgE did not attain statistical significance (p greater than 0.05) in the various tests carried out. After incubation with H1 receptor antagonists an inhibition curve of the histamine release in the presence of Dermatophagoides pteronyssinus was obtained. The inhibition of this curve on day -1 was higher for terfenadine and mequitazine than for azatadine and astemizole.(ABSTRACT TRUNCATED AT 250 WORDS)