There has been growing evidence to support the possible physiological role of calcium-activated chloride channels (Cl_(Ca)) in neurotransmitter-induced contractile responses in smooth muscle (Large & Wang, 1996; Kitamura & Yamazaki, 2001). Niflumic acid (NFA) reduces the contractile responses produced by noradrenaline in rat aorta (Criddle et. al., 1996) and 5-HT in rat stomach fundus (Scarparo et. al., 2000) by blocking (Cl_(Ca)). The aim of the present study was to investigate the role of (Cl_(Ca)) in carbachol (CCh)-induced contractions of longitudinal smooth muscle from rat isolated distal colon using NFA. Male Wistar rats (275 – 300 g) were euthanased by cervical dislocation and exsanguination. A segment of distal colon was excised, everted and the mucosa and submucosa removed from the muscularis. Muscle rings of about 1.0 cm length were mounted vertically under 1 g tension in 3 ml siliconised organ baths containing Krebs solution (composition, mM: NaCl, 118; KCl, 4.74; CaCl₂,2.54; KH₂PO₄, 1.19; MgSO₄.7H₂O, 1.20; NaHCO₃, 25; glucose, 11) maintained at 37 °C and bubbled with 95% Oand 5% CO₂.Preparations were allowed to equilibrate for at least 60 minutes prior to drug addition. Maximal responses to CCh (10 µM) and KCl (120 mM) were recorded before and after incubation with increasing concentrations of NFA (1 – 300 µM) for 30 minutes. The effects of 3 µM nifedipine, a voltage-dependent calcium channel blocker, were also tested. Values are mean ± s.e.m. Statistical analysis was carried out using Student’s t-test, p<0.05 is significant.Contractile responses to CCh were reduced in a concentration-dependent manner after incubation with NFA giving a pIC₅₀ value of 4.0 ± 0.1 (n = 4 – 12).The highest inhibition observed was 84.5 ± 2.1 % (n = 5) with the incubation of NFA at 300 µM. KCl-induced contractile responses were unaffected (n = 5). Incubation with 3 µM nifedipine resulted in a significant reduction of the contractile responses of both CCh (62.1 ± 7.1 %, n=8) and KCl (85.4 ± 2.2 %, n=8). The combination of nifedipine and NFA (100 µM) had no greater inhibitory effect on CCh responses than nifedipine alone which also supports that the effect of NFA is on (Cl_(Ca)). The differential effects of NFA on CCh and KCl supports the involvement of (Cl_(Ca)) in CCh-induced contractions in the longitudinal muscle of the rat colon.