Concentrations of glucose, growth hormone, insulin, and glucagon in blood plasma relative to days in milk, milk production, type of housing, and season were measured. Blood samples were obtained from 133 to 150 lactating Holstein cows on 3 consecutive days in July, October, January, and April. Glucose, insulin, and ratio of insulin to glucagon increased with increasing days in milk. Growth hormone and ratio of growth hormone to insulin decreased with increasing days in milk. Glucagon concentrations were similar throughout lactation. Above average milk production throughout lactation was associated with above average glucagon, lower insulin, and lower glucose. The relationship was not significant between growth hormone concentration and milk production, however. Cows on summer pasture with limited grain supplement had higher growth hormone and lower glucose, insulin, and glucagon than cows eating ad libitum in barns or feedlot. In general, however, all cows had higher glucose and lower growth hormone, insulin, and ratio of insulin to glucagon in July than in cooler months.