A lot of publications are about the chemical influence of mouthrinses on dental plaque. Most of this investigations are in vitro methods of testing. Ultrastructure analysis of dental plaque are seldom published and mostly done with a scanning electron microscope. The aim of this study is a quantitative comparison between effects of different antimicrobial mouthrinses on dental plaque in vivo. This quantitative investigation was realized with the method of transmission electron microscopy (TEM). 21 adults aged 18 to 62 years participated randomized in a three days study. The subjects wore plastic splints with three pieces of bovine enamel in the molar area of the lower jaw. An amine fluoride / stannous fluoride mouthrinse (ASF) in group 1, 0.12 % chlorhexidine digluconate mouthrinse (CHX) in group 2 and 0.7 % sodium chloride mouthrinse (SCL) in group 3 have been used for rinsing. The rinsing procedure was executed with 10 ml mouthrinse for 30 seconds twice a day. 24 (a), 48 (b) and 72 (c) hours after baseline each a piece of enamel with the plaque layer was removed and prepared for TEM. Ultrathin slices were inspected relating to plaque thickness (PT in µm), quantity of cocci (QC per 10 µm2) and quantity of rods (QR per 10 µm2). Friedman-/Wilcoxon-Test was used for statistical analysis. The results for group 1 are: PT/a 8.42±3.66, PT/b 11.65±5.27 (p=0.018), PT/c 14.02±6.37 (p=0.018); QC/a 15.68±6.16, QC/b 18.08±9.50 (n.s.), QC/c 12.04±5.37 (p=0.018); QR/a 0.23±0.19, QR/b 0.81±1.23, QR/c 0.52±0.44 (n.s.). The results for group 2 are: PT/a 6.91±5.97, PT/b 9.98±6.27 (p=0.018), PT/c 11.91±7.00 (p=0.018); QC/a7.02±4.01, QC/b 7.52±5.27 (n.s.), QC/c 4.29±3.76 (p=0.018); QR/a 0.21±0.47, QR/b 0.25±0.22, QR/c 0.08±0.13 (n.s.). The results for group 3 are: PT/a 4.69±1.88, PT/b 10.60±5.31 (p=0.018), PT/c 19.24±5.70 (p=0.018); QC/a 14.25±3.78, QC/b 16.98±2.82 (p=0.018), QC/c 18.18±2.60 (p=0.018); QR/a 0.52±0.18, QR/b 0.88±0.19 (p=0.018) QR/c 1.29±0.37 (p=0.018). All groups showed significant changes at PT and QC, group 3 showed additionally a significant increase of QR. The results permit the conclusion that the effects of ASF and CHX relating to plaque thickness and quantity of cocci as wel as start of efficacy are very similar. Keywords: mouthrinses, dental plaque, transmission, electron microscopy