The basement membrane models which have been proposed to date are generally based on biochemical data, mainly binding studies and artificially synthesized polymers in vitro. Basically these have led to models proposing two three-dimensional laminin-1 and collagen type IV networks interconnected by nidogen-1. Whether they reflect the in vivo basement membrane structure is still not clear. We localized laminin-1, nidogen-1, and collagen type IV ultrastructurally in adult and fetal mouse kidney basement membranes with the help of immunogold-histochemistry performing double and triple localization to try to elucidate the molecular organization of basement membranes in vivo. We found laminin-1, nidogen-1, and collagen type IV distributed over the entire basement membranes in adult and fetal kidneys. This contradicts earlier studies ascribing laminin-1 to the lamina lucida and collagen type IV to the lamina densa. In addition, various basement membrane segments exhibited an organized labeling pattern for the BM components. Double-labeling revealed co-localization of laminin-1 and nidogen-1. We conclude that the combination of laminin-1 with collagen type IV as double-network basement membrane partially interconnected by nidogen-1 is found already in the early fetal kidney in vivo. However, our data cannot exclude the possibility of other variants of basement membrane assemblages. This is also indicated by a changing structure even in individual segments of one basement membrane type which renders a more flexible basement membrane architecture plausible.