Changes in regional microvascular albumin flux during septic shock were studied noninvasively by scintigraphy in the baboon model. Use was made of an i.v. injection of 99mTc-labeled baboon serum albumin. Count ratios of lung to cardiac, liver to cardiac, and abdominal to cardiac regions were measured two-hourly for 6 hr in six control and six septic shock baboons (live E. coli) and compared. Increased ratios obtained during shock pointed to an increase in extravascular albumin. Linear regression lines fitted to these count ratios provided regional albumin leak indices. These indices demonstrated statistically significant increases (P less than 0.05) during septic shock for the abdominal region during the 6-hr study, and for all regions, but especially the abdomen, when data were calculated over 4 hr. Increasing ratios and leak indices correlated with postmortem data and with changes in neutrophil and platelet behaviour previously established during shock. Possible accompanying mediator releases could be responsible for the endothelial damage leading to the increased permeability.