Thirty-two 6--8-month-old Southdown ram hoggets (16 each from the Massey University High and Low Backfat selection lines) were housed in metabolism crates and offered lucerne chaff at 1.2 x maintenance requirements. Daily urine output was measured and urine sampled over a 7 -day balance period (bulk sample) and on each of the last 3 days. Rams were fed at 2-hourly intervals for the last 24 h of the period and jugular blood samples withdrawn before each feed. Rams were then fasted for 64 h before being refed, with blood samples being taken throughout fasting and refeeding. Low Line rams maintained significantly lower concentrations of urea (but not of creatinine) in plasma than High Line rams throughout the 24-h sampling, fasting, and refeeding. Plasma glucose concentrations were lower in Low Line rams during fasting but increased more rapidly in this group after refeeding. The elevation of insulin concentrations which occurred after refeeding was also greater in Low Line than in High Line rams. Plasma concentrations of non-esterified fatty acids and growth hormone were not influenced by line. During the 24-h sampling periods and the fast, weight -corrected urea clearance rate was greater in the Low Line than in the High Line rams. Plasma urea concentrations were independent of age in the lReprint requestsReceived 13 September 1988; accepted 30November 1988 Low Line rams but declined with advancing age in the High Line rams. It is concluded that differences between the lines in plasma urea concentration are largely determined by differences in urea clearance, but that this cannot be accounted for entirely by differences in glomerular filtration rate.