Pancreas transplant results have been better in uremic recipients of a simultaneous kidney than in nonuremic recipients of a pancreas alone. We studied the relative effect of uremia versus a double transplant on functional survival by performing bladder-drained pancreas transplants alone (PTA), kidney transplants alone (KTA), and simultaneous pancreas/kidney (SPK) transplants from Buffalo donors to diabetic Lewis rat recipients that were or were not made uremic 2-3 weeks before by 1 4/5 native nephrectomy. Pancreas graft exocrine function was monitored by urinary amylase (UA). In the PTA and SPK recipients made diabetic by streptozotocin, endocrine function was monitored by measuring nonfasting plasma glucose (PG) levels. Kidney graft function was monitored by plasma creatinine (Cr). Rejection of the endocrine pancreas was defined as an increase of PG to greater than 200 mg/dl; of the exocrine pancreas, as a decline in UA to less than 6000 U/L or to less than 100 U/24 hr; and of the kidney, as an elevation of Cr to greater than 3 mg/dl. The mean functional survival times (MST) of both the endocrine (12.0 +/- 2.1 versus 10.1 +/- 1.1 days, P = 0.036) and exocrine (8.0 +/- 2.1 versus 6.3 +/- 1.3 days, P = 0.016) components of the pancreas grafts were significantly longer in SPK than in PTA recipients. The MST of kidney allografts, however, was not significantly longer in nonuremic SPK than nonuremic KTA recipients (6.7 +/- 1.4 versus 5.7 +/- 0.7 days, P = 0.13). In parallel experiments in recipients immunosuppressed with cyclosporine, the graft survival times were longer, but the relative differences between the PTA, SPK, and KTA groups persisted. Histologically, lymphocyte infiltration began in the two organs almost simultaneously, but the severity of the rejection was more vigorous in the kidney than in the pancreas in doubly grafted rats, and destruction of pancreas grafts progressed more slowly in SPK than in PTA recipients. Preexisting uremia delayed pancreas rejection in both SPK (exocrine 10.6 +/- 2.3, P = 0.032, and endocrine 14.8 +/- 3.4 days, P = 0.065, versus nonuremics) and PTA (exocrine 8.5 +/- 1.7, P = 0.007, and endocrine 12.6 +/- 2.5, P = 0.026, versus nonuremics) nonimmunosuppressed recipients. The MST of kidney grafts was not significantly longer in uremic (8.9 +/- 2.8 days) than in nonuremic (6.7 +/- 1.4 days) SPK recipients (P = 0.081). A synchronous kidney transplant and uremia independently down-modulate the rejection response to a pancreas graft, and a simultaneous pancreas graft has no detrimental effect on the survival of a kidney graft.(ABSTRACT TRUNCATED AT 400 WORDS)