This is an interim report on the lifetime study of chronic mortality and its causes under investigation in 31 control (20 males, 11 females) and 217 survivors (124 males, 93 females) of an acute 90-day experiment in rhesus monkeys. Single acute whole-body exposures were made using 32-, 55-, 138-, 400-, and 2300-MeV protons in 1964-1965. Doses ranged from 25 to 800 rad and dose rates from 12.5 and 100 rad per minute. Tissue depths of partially penetrating 32- and 55-MeV particles were approximately 1 and approximately 2.5 cm, respectively, and depth doses at the respective distances were 115 and 122% of surface doses. Protons with energies greater than or equal to 138 MeV were totally penetrating and the depth doses were essentially homogenous. For pooled data: (1) mortality was significantly higher (P less than 0.01) in irradiated animals (48%) than in controls (19%); (2) mortality in animals exposed to partially penetrating 55-MeV protons (53%) was essentially similar to those given totally penetrating 138- (53%), 400- (49%), and 2300-MeV (44%) exposures; (3) proton energies and doses that were effective in producing life shortening were greater than or equal to 55 MeV and greater than or equal to 360-400 rad, respectively; (4) death rates for irradiated animals compared to controls began to increase after approximately 8 years, approximately 2 years, and approximately 1 year for those exposed to 360-400, 500-650, and 800 rad, respectively; (5) of the nine probable causes of death reported, the leading causes were primary infections in both irradiated (31%) and control (50%) animals, endometriosis (25% vs 0%, respectively), neoplasms (17% vs 0%), and organ degeneration (17% vs 33%); and (6) if endometriosis is included with the neoplastic group, deaths from all forms of neoplasms would be 42% in irradiated animals.(ABSTRACT TRUNCATED AT 400 WORDS)