The consequences of peri-parturient Monensin administration were studied in two dairy herds in ≥ 2 parity HF cows (1) calving in moderate body condition (score, BCS: 3.25-3.75) and producing ≥8500 kg FCM in the previous lactation (Herd K), as well as (2) in those being overfed before calving (BCS: ≥3.75), but producing less milk (5-8000 kg FCM) (Herd S). In both herds on days 249-256 of gestation half of the cows received an intraruminal constant release capsule of Monensin (Rumensin® capsule, ELANCO) (Monensin-K and Monensin-S cows), the others remained untreated controls (Control-K and Control-S cows). Blood samples were taken on day 1-3 after calving and again 4 times with 7 days intervals to assay certain hormones and metabolites. On days 1-3 and again on days 28-35 also the ACTH induced Cortisol response and TRH induced T4/T3 responses were determined. The resumption of ovarian cyclicity was followed by milk progesterone profiles. In the first 10 weeks after calving the milk production of Monensin-K cows (n=13) did not differ from that of Control-K cows (n=14). The cows lost their body weight continuously in the first 10 weeks of lactation in Herd S, whereas only in the first 5 weeks in Herd K. Monensin only slightly influenced the degree of weight loss (on both farms), and the circulating leptin level (determined only in Herd S). Significantly lower insulin, IGF-1, T4, T3, glucose and cholesterol levels as well as less elevated βOH-butyrate and non-esterified fatty acid concentrations were seen in Control-S (n=13) than in Monensin-S (n=ll) cows, first of all in the first two weeks after calving. These ionophor-related differences were less obvious or disappeared in Herd-K. Monensin shortened the postpartum acyclic and anestrous periods as well as the calving to re-conception interval but this effect was significant only in the Herd S.