Cells with irregular bodies and numerous thin long processes were described in resistance arteries (Pucovský et al., 2003). Our recent results (Pucovský et al., 2005) have shown that they are immature or phenotypically modulated myocytes. The aim of this work was to establish whether these cells are constitutively present in the vessel wall or are mature myocytes that have modulated their phenotype only after being isolated from the tissue. The experiments were carried out on guinea-pig mesenteric artery segments and on single cells isolated from them by enzymatic digestion. The cells were labelled by incubation with primary antibodies against a target molecule and the labelling was visualised using fluorescent secondary antibodies. The images were obtained with a laser scanning confocal microscope. Vessel segments used for electron microscopy were first placed in physiological solution containing 100 µM nicardipine for 3 hours and then were processed, cut in sections and viewed with transmission electron microscope. Statistical significance was tested with Student’s t-test. The fluorescent signal intensity of myosin light chain kinase in immature myocytes was 32.1±6.6% of that in mature myocytes (n=9 cell pairs, p=10-5), which is in accordance with their inability to contract. Cell isolation in the continuous presence of 20 µM latrunculin B, actin polymerisation inhibitor, did not cause the disappearance of cells with processes from cell suspension, suggesting that some cells had processes even before isolation. The fluorescence signal of basal lamina protein collagen IV in immature myocytes was 113.3±12.1% of that in mature myocytes (n=9 cell pairs, p=0.305), but the laminin fluorescence was significantly higher in immature myocytes, 131.9±9.6% of that in mature myocytes (n=8 cell pairs, p=0.013), ruling out phenotypic modulation due to basal lamina shedding. The cells with thin processes were found in the tunica media of small resistance arteries using transmission electron microscopy. The results suggest that myocytes of immature phenotype are constitutively present in resistance arteries of guinea-pig.