Dietary casein, compared to vegetable protein, causes hypercholesterolemia in some animal species. This may be associated with a change of hormonal status. Among others, GH has an important impact on cholesterol metabolism; GH deficiency results in hypercholesterolemia. This paper shows that the rhythmic variation of GH levels in rats is differently affected by different dietary proteins. Within a 4-h observation period overall mean values and integrated areas under the GH levels plotted against time are higher with casein as compared to soy protein. Secretory GH peak values are lower than reported before for chow-fed rats. These observations support the idea that different dietary proteins cause a different endocrine response. As GH levels are higher with casein, while lower levels would be expected to be associated with hypercholesterolemia, the observed differences are obviously of less relevance for the expression of casein-induced hypercholesterolemia.