Aspergillus endocarditis in man usually occurs on prosthetic cardiac valves and gives rise to large vegetations which embolize easily producing peripheral organ infarction and infection. Blood cultures are usually sterile and the disease is difficult to cure with antimicrobial agents. Aspergillus endocarditis was studied in rabbits to determine the course, degree of fungemia, and response to treatment with amphotericin B (A), 5 flucytosine (5 FC) or A + 5 FC. Polyethylene tubing was introduced into the left ventricle through the carotid artery and 24 hours later animals were inoculated with 10(4) to 10(7) spores of a strain of Aspergillus fumigatus. Large occlusive vegetations developed on the aortic valves. Spontaneous mortality reached 67 per cent after 3 days. Despite large aggregates of mycelia seen beneath a layer of amorphous material on microscopic sections, vegetations contained only 10(3) to 10(5) colony forming units (CFU) of aspergilli per gram, suggesting the aspergilli in tissues were clumped. Disseminated infection involving kidney, lung, liver, spleen, and brain occurred. Animals without intracardiac tubing which received the same inoculum of spores did not develop endocarditis, but showed evidence of disseminated infection. Blood after 24 hours of infection grew aspergilli only when large volumes were cultured and then only a small fraction of the total volume of blood obtained for culture yielded aspergilli, suggesting that the aspergilli in blood were clumped. Sterile vegetations in the absence of an intracardiac catheter were resistant to infection with aspergilli, but once established, infection with aspergilli persisted on vegetations despite removal of the catheter. Treatment of infected animals with A (1 mg. per kilogram), 5 FC (25 or 50 mg. per kilogram) or A + 5 FC daily intraperitoneally, significantly lowered the number of CFU per gram of vegetation.