Blood samples taken from 86 Holstein cows in various stages of lactation at 4 h after feeding concentrates were analyzed by radioimmunoassay for insulin, thyroxine, triiodothyronine, and prolactin. The circulating insulin and thyroxine were correlated positively with stage of lactation and negatively with daily milk yield. The circulating triiodothyronine was correlated positively with thyroxine but was not correlated significantly with stage of lactation or daily milk yield. The circulating prolactin was not correlated with either stage of lactation or milk yield. In a multiple regression analysis with several independent variables, more than 60% of the accountable variation in daily milk yield was due to lactation stage and an additional 30% to age of cow. Within lactation stages, simple regressions of daily milk yield on circulating insulin, thyroxine, and triiodothyronine were not significant. However, during the first 60 days of lactation daily milk yield increased 6 kg per ng/ml of circulating prolactin.
