In order to examine the influence of the clock hour at which an acute inflammatory reaction is induced on the kinetics of both corticosterone and haptoglobin hyperproduction, turpentine oil was administered sc to female rats (0.5 ml⁄100 g body wt), kept under a 12 hr light- 12 hr dark light regimen, at 09 hr, 13 hr, 21 hr and 03 hr. In each experiment 15 groups of 5 animals were sacrificed at time intervals ranging from 10 min to 120 hr after injections (a.i.). The plasma samples were assayed for corticosterone (C) by an improved fluorometric method, and for haptoglobin (Hp) by the peroxidatic activity of a haptoglobin-hemoglobin complex (Jayle, 1951). The C hypersecretion exhibited in the 4 groups a biphasic pattern. The first, stressinduced C increase attained in all groups the same ceiling level (about 100 µg⁄100 ml). However the time lag a.i. of the C peak varied from 20 min. in the 15 hr or 21 hr-injected rats, to 1 to 2 hr in the 03-hr stressed animals. The secondary, long-lasting C augmentation started at 4 to 8 hr a.i., and was correlated with the Hp hyperproduction reflecting the inflammatory reaction. The Hp reaction of the 4 groups differed in that the 09-hr injected rats exhibited an early increase (4 to 6 hr a.i. w 12 hr in the other groups) associated with a higher maximum, whereas in the 03 hr-injected animals the HP increased only slowly, to reach the lowest maximum among the groups. These results are consistent with the concept of an adrenocortical control of haptoglobin biosynthesis. They also stress the importance of the timing in the occurrence of a stress for the speed and amplitude of physiological reactions. (Endocrinology94: 284, 1974)