In women with renal transplants glomerular filtration rate (GFR) increases during pregnancy but how soon the increment occurs, its relation to pre-pregnancy GFR, and the overall pattern of change are unknown. Twenty-four hour creatinine clearance (24-hr CCr) were measured prospectively in ten pregnancies in eight allograft recipients before conception, throughout pregnancy, 8 to 12 weeks postpartum, and 4 to 6 monthly thereafter. Inulin (CIn) in creatinine (CCr) clearances during infusion were also determined and protein excretion was evaluated. The results were compared to those in similar studies in ten healthy women. By the tenth gestational week 24-hr CCr was 124 +/- (SD) 15.9 ml/min in healthy women (an increase of 38%; range, 18 to 69%) and in transplant patients was 105 +/- 28.1 ml/min (an increase of 34%: range, 10 to 60%), with the greatest increments in those whose allografts functioned best before conception, regardless of donor source and sex or the transplant-pregnancy interval. In late pregnancy mean 24-hr CCr decreased by 19% (range, 6 to 28%) in healthy women and by 34% (range, 12 to 57%) in the transplant patients, but in most this did not represent graft deterioration nor lead to permanent impairment. At all time points CIn values were 5 to 10% greater than those for 24-hr CCr but slightly less than infusion CCr values. Protein excretion increased throughout pregnancy and by the third trimester in healthy women averaged 200 mg in 24 hr and regularly exceeded 500 mg in 24 hr in transplant patients, which was three times non-pregnant levels and probably not clinically significant.(ABSTRACT TRUNCATED AT 250 WORDS)