Histamine, a primary mediator of the inflammatory response, is a potent spasmogen of smooth muscle. Sources of activator Ca²⁺ for histamine-induced contraction of avian intestinal smooth muscle have not been described. The aim of this study was to assess the contribution of intracellular and extracellular Ca²⁺ to histamine-mediated contractions. Male Cobb 500 chickens (aged 21-42 days) were humanely killed using CO₂ asphyxiation. Full thickness longitudinal intestinal smooth muscle strips were suspended under 1 g of tension, in a physiological buffer at 37°C gassed with 95% O₂ /5% CO₂. Isometrically measured contractile responses were expressed as a % of the response to carbachol (CCh, 100μM). Statistical analyses were made using Student′s unpaired two-tailed t-test or analysis of variance. Cumulative addition of histamine (0.01-100 μM) caused rapid onset, concentration-dependent, sustained contractions of muscle strips (log EC₅₀ = -5.4 ± 0.2) with the peak tissue responses reaching 76 ± 9% of the maximum CCh response (mean ± SEM, n = 28). The histamine concentration-response curve was sensitive to mepyramine (1 μM) but not cimetidine (1 μM) nor thioperamide (1 μM), indicating that histamine primarily acts via H1 receptors. Histamine-induced contractions were abolished in a nominally Ca²⁺-free medium containing 1mM EDTA (p〈0.005, n=5). Pre-treatment with the L-type voltage-operated Ca²⁺ channel blocker nifedipine (1μM) caused a significant reduction in basal tone (p〈0.05, n=5) but had no effect on histamine-induced contractions. Pre-incubation with SK&F 96365 (50μM), a blocker of both receptor-activated and voltage-operated Ca²⁺ channels, significantly reduced the peak contractile response to histamine (25 ± 9%, p〈0.0005, n=6) and also reduced initial resting tension (p〈0.05). The selective sarcoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin (1μM), used to deplete intracellular Ca²⁺ stores, had no effect on basal tone or on histamine-mediated contractions (n=5). In conclusion it would appear that histamine-induced contractions of chicken small intestine smooth muscle are heavily dependent on extracellular Ca²⁺ influx via receptor-activated Ca²⁺ channels. Intracellular Ca²⁺ release appears to have little role to play in histamine-mediated contractions. The basal tone of the tissue is both SK&F 96365 and nifedipine sensitive, suggesting that different Ca²⁺ channels may contribute to the maintenance of resting tone and histamine-mediated contractions.