Spontaneous changes of diameter in small arteries and arterioles were studied in the hamster skin fold chamber preparation, which allows to observe the microcirculation without acute exposure and anesthesia. Continuous measurements of diameter were made when the preparation appeared to be physiologically normal according to preset criteria. 89% of the arterial vessels studied showed spontaneous changes in diameter. The activity was prevalently periodic with 29% of the vessels exhibiting random changes in diameter. The amplitude of the periodic activity expressed as a percentage of the mean diameter increases in proportion to the vessel size, arterial vessels in the 40-100 micron diameter range exhibit amplitudes of the order of 10-20%. The smallest arterioles show changes as large as 100% of mean diameter, which cause these vessels to become temporarily occluded. This phenomenon is analyzed in terms of its effect on peripheral vascular resistance by means of an analytical model that assumes vasomotion to be a periodic phenomenon. It is shown that the presence of time dependent changes in vessels diameter similar to those observed, has an important effect on peripheral vascular resistance.