Concentrations of LH, FSH, prolactin (PRL), estradiol-17β (E2), and progesterone (P) were measured in samples of peripheral blood, which were taken without stress from four intact pigs every hour for a 100 h period during the expected time of estrus, and a 24 h period during Days 10 and 11 of the estrous cycle. Additional blood samples were taken once daily during the whole estrous cycle. A preovulatory LH surge was observed with a duration of ∼28 h. The start of this LU surge preceded the time of maximum E2 concentrations by 1-3 h. The E2 maximum in turn preceded the maximum concentration of LH by 8-15 h. During proestrus, rising E2 levels were accompanied by a decrease of FSH. At estrus, a small FSH surge was observed coincident with a rapid fall of E2 and approximately parallel to the surge of LH. A second rise of FSH started at about 27 h after the time of maximum LH concentration and was accompanied by a rise in progesterone. This rise of FSH culminated in a peak at Day 3 of the cycle, on the average, which coincided with a minimum concentration of E2. Two separate PRL surges were observed, one during the rise of the proestrous E2 peak and one during estrus. Estrous behavior had a mean duration of 53 h, and practically coincided with a period of elevated plasma PRL concentrations. During the luteal phase, pulsatile secretion patterns were observed for LH, and to a lesser degree also for FSH, E2, P, and PRL. Most prominent parallelism was observed between pulses of LH and E2, with E2 in most cases following LH after a latent period of ∼1 h. A diurnal rhythm was observed only for PRL, with minimal plasma concentrations during the late night.