Isolated endothelial cells (n 110; 350-540 microns3 in volume) of the sinusoid of rat liver were found to differ from in situ endothelial cells by their rounded shape with the nucleus usually in the centre and fenestrae of the sieve plates across the entire cytoplasm. Their volume density is 0.2814 (SE 0.0085). Some of them tend to form complex, ramified structures as well as clefts and vacuoles up to 1 micron in size. Dense bodies are other structures quite often seen in endothelial cells (Vv 0.0189, SE 0.0085). Mitochondria, on the other hand, are extremely rare (Vv 0.0023, SE 0.0009). Both the preservation and relocation of sieve structure after isolation may probably be due to the cytoskeleton which in vivo causes changes of size and number of endothelial cell fenestrae under physiological and pathological conditions.