Dairy cows were fed from the start of lactation either low (group LP) or normal amounts of protein (group NP), but similar amounts of energy. In the 3rd or 4th wk (period I) and 11th or 12th wk of lactation (period II) digestion trials were performed and nitrogen (N) balances determined. Immediately afterwards changes of hormones and metabolites were studied in blood for 24 h. In period I balances of absorbable protein (AP) and N of group LP were markedly inferior than in group NP (p < 0.05), while energy balances were similarly negative. In period II balances of AP, N and energy were similar in group LP and NP. Differences occurred between periods I and II with respect to balances of N, AP and energy (p < 0.05). Fat-corrected milk yields in both groups in periods I and II were similar. Milk protein and milk urea contents were however lower in period I in group LP than in group NP (p < 0.05). Of metabolites and hormones in blood only urea and partially alpha-aminoacid-N and in period II only 3-methylhistidine were different between groups (p < 0.05), whereas concentrations and behaviour of glucose, non-esterified fatty acids, acetoacetate, beta-hydroxybutyrate, protein, albumin, insulin, glucagon, somatotropine, insulin-like growth factor I, cortisol, thyroxine and triiodothyronine were similar. However and likely in association with the improved energy supply, concentrations of non-esterified fatty acids (groups LP and NP), beta-hydroxybutyrate (group NP) and urea (group NP) decreased from period I to period II, while levels of insulin (group LP), insulin-like growth factor I (groups LP and NP), thyroxine (group LP) and triiodothyronine (group NP) increased (p < 0.05).