Circadian changes in protein synthesis were studied in lactating rats fed diets varying in protein quality and feeding level. At parturition, rats previously fed a stock diet were assigned to semipurified diets supplying 23% protein. Two groups were fed wheat gluten (WG) or casein (C) ad libitum; a third group was pair-fed casein to consumption of the WG group (C-PF). At four different times on day 15 of lactation, protein synthesis was measured in vivo in mammary gland, liver and calf muscle with a large dose of L-[4-3H]phenylalanine. Pup weights at day 15 for dams fed WG, C and C-PF were 19.3 +/- 0.3, 33.9 +/- 0.5 and 27.2 +/- 0.5 g, respectively. No significant circadian variation in fractional synthesis rate (FSR) was observed in either mammary gland or liver. Higher variability in muscle FSRs precludes conclusions in regard to this tissue. However, protein quality and level of feeding both affected FSR. For rats fed WG, C and C-PF, FSRs were, respectively: 77 +/- 4, 116 +/- 6 and 64 +/- 3%/day in mammary gland; 67 +/- 4, 78 +/- 5 and 74 +/- 4%/day in liver; and 3.4 +/- 0.5, 6.7 +/- 1.0 and 4.0 +/- 0.8%/day in muscle. Absolute synthesis rates were, respectively: 584 +/- 60, 1743 +/- 148 and 663 +/- 58 mg/day in mammary gland; and 469 +/- 46, 970 +/- 72 and 702 +/- 62 mg/day in liver. The results confirm that dietary protein quality and feeding level affect protein synthesis rates in lactating rats and demonstrate that there is no circadian variation in protein synthesis in mammary gland and liver of lactating rats.