1. In the present study we investigated desensitization phenomena of guinea-pig jejunal longitudinal smooth muscle responses after stimulation with 100 microM histamine or methacholine, using a superfusion method. 2. Histamine H1-receptor-mediated contractions appear to be rapidly reduced after application of 100 microM histamine. Muscarinic responses were not affected following desensitization with 100 microM histamine, indicating a homologous desensitization. 3. Initial contractions to 0.3 microM histamine were reduced by 90%, recovered quickly, but did not reach control levels within 1 h. Desensitization of histamine responses could be separated into two phases; a rapid, but transient, desensitization and a more sustained desensitization. As a consequence of this sustained effect the pD2 for histamine shifted from 6.7 +/- 0.1 (control) to 6.1 +/- 0.1 (desensitized). 4. Desensitization with 100 microM methacholine caused a heterologous desensitization, reflected by the development of a refractory period, in which neither histamine nor methacholine was able to elicit a contraction. After a few minutes responses to both agents recovered to control levels. 5. During the refractory period after methacholine desensitization, muscle strips were still responsive to 40 mM KCl but did not contract in response to 10 mM caffeine, suggesting that the heterologous desensitization is caused by a modification of an intracellular Ca2(+)-store, which is used by both histamine and methacholine. 6. The recovery of the responses after methacholine desensitization was not dependent on extracellular Ca2+, suggesting that the recovery is not dependent on refilling of the intracellular Ca2+ store with extracellular Ca2+. 7. The protein kinase C activator, phorbol-12,13-dibutyrate, concentration-dependently inhibited histamine- and methacholine-induced contractions. Protein kinase C seems therefore not to be implicated in the observed homologous H,-receptor desensitization. 8. These data suggest that different forms of desensitization can be distinguished in this model, each with a different time course and dependent on the applied stimulus.