In this study of normal adult human and ovine ureters, a characteristic distribution of a large population of typical mast cells was described by light and electron microscopy. Pharmacological studies were used to ascribe a functional role for these cells in normal and pathological states. In the structural investigations typical mast cells with their cytoplasm packed with characteristic electron dense granules were found in close vicinity to smooth muscle cells. A close association between mast cells and a fibroblast like La-cell and non myelinated nerve fibers was noted. The prevalence of mast cells was higher in human ureters. Human and sheep ureteral ring preparations exhibited spontaneous rhythmical contractions in vitro. Addition of histamine (10(-6)-10(-5) M) induced an increase in the frequency of contractions and enhanced the basal tone particularly in human samples. It is likely that histamine under pathological conditions such as renal colic and inflammatory reactions is released from mast cells within the ureter and induces a state of forceful contractions and pain fibre stimulation.