The presence of interstitial cells (ICs) of Cajal, or morphologically similar c-kit-positive/c-kit-negative cells in a range of visceral and vascular tissues throughout the bodies of different species has been reported in a number of recent publications [1]. The present study provides initial data about the presence of ICs in the wall of branches of the mouse mesenteric artery (MMA) and about some features of single ICs freshly isolated from this blood vessel. Experiments were performed on segments (about 2 mm in length) of branches of MMA (from 1st to 5th order) from humanely killed BALB/C mice and on single cells from these within a few hours of enzyme and mechanical dispersion. MMA segments were mounted on glass pipettes, secured by 10-0 sutures, pressurized to 60 mmHg and then the living tissue was stained with Phycoerythrin-Cy7 conjugated anti-mouse CD117 (c-kit) antibodies (eBioscience) or with rat anti-mouse primary monoclonal c-kit antibodies (RDI), followed by Alexa Fluor 633 conjugated goat anti-rat secondary antibodies (Molecular Probes). In some experiments electrical field stimulation (EFS; typical parameters: 0.5-10 Hz, 0.5 ms, 50-60 V) was applied via two platinum wires placed parallel to the long axis of unstained arterial segments, loaded with the fluorescent Ca²⁺-sensitive indicator fluo-4 AM (Molecular Probes). Single cells were imaged also after loading with fluo-4 AM. ICs were detected using laser scanning confocal microscopy in the adventitial layer of MMA, as was suggested to be the case for the human cerebral arteries [2]. Only larger branches (> 200 μm) showed cells with strong positive c-kit staining, while smaller ones did not stain. Confocal microscopy of fluo-4-loaded pressurized segments of this blood vessel revealed junctional Ca²⁺ transients (similar to [3]: mean peak ratio F/F₀, 2.74; FWHM, 5.4 μm; t_peak, 80 ms; t_1/2, 510 ms) accompanied sometimes by contraction in response to low-frequency EFS only in the case of large branches, but not the smaller ones. X-Y fluorescent confocal imaging of single fluo-4-loaded cells enzymatically isolated from the whole MMA tree showed (among numerous myocytes) a small population (about 5%) of branching cells morphologically closely resembling ICs of Cajal, widely described in the gut [4]. Similar to the recently described rabbit portal vein ICs [5], ICs freshly isolated from the MMA varied in their morphology (from 18 to 62 μm in length) but always had numerous thin processes up to several tens of microns in length, and, unlike myocytes, these cells did not contract in response to noradrenaline (1-10 μM), caffeine (1-10 mM) or high K⁺ solution (60 mM), although they responded with an increase in [Ca²⁺]_i. The functional role of ICs in the MMA is not yet clear; they may play a role as intermediaries between the nerves and the smooth muscle cells.