Glycogen distribution has been examined in placental capillaries in normal (N; 11), overtly diabetic (OD; 8) and gestational diabetic (GD; 7) pregnancies using light and electron histochemistry. Light microscopy revealed a wide variation in glycogen deposition in all three groups, but the method was too insensitive to visualize deposits in the villous capillaries. An ultrahistochemical technique, however, rendered glycogen detectable. Endothelial cells (EC) and pericytes (PERI) in 100 capillaries per specimen were scored as to whether their glycogen was dispersed (D) or aggregated (A). Again, a large range in the amounts of both types of glycogen was seen. When EC and PERI comprising a capillary cross-section were considered together as 'functional units', a higher (P < 0.11) relative number of cells containing A was found in OD (51 +/- 8 per cent) than in N (41 +/- 17 per cent) or GD (43 +/- 18 per cent). In OD more EC and PERI, considered separately, tended to contain A than those in N and GD, but large variations in the data precluded statistical significance. The proportion of PERI containing aggregated glycogen was higher (P < 0.0002) than that of EC in all groups. We suggest that, on average, placental EC and PERI contain higher glycogen levels in OD than in N or GD. Moreover, these cells might contribute to the high glycogen levels in placentae from diabetic pregnancies.