The effects of +GZ acceleration stress on renal blood flow were studied in chronically instrumented, conscious miniature swine. Renal blood flow was determined using the radiolabeled (46Sc, 85Sr, 141Ce, and 125I) microsphere technique (29 animals) and Doppler-principle flow probes (1 animal). The results obtained with the microsphere technique indicate that +GZ exposure with anti-G suit support caused decreases in renal blood flow throughout the kidney, and that renal blood flow remains decreased for 1-10 min after +GZ stress. Medullary blood flow appeared to recover after +GZ exposure more rapidly than cortical blood flow. The flow probe data also indicate that +GZ stress with or without anti-G suit support results in decreases in renal blood flow, and that anti-G suit inflation in the absence of +GZ can also cause significant decreases. We conclude that the reductions in renal blood flow observed in miniature swine, during +GZ stress with anti-G suit support, may be due to a combination of direct anti-G suit interference and +GZ-induced reflex increases in alpha-constrictor sympathetic tone to the renal vascular bed.