The three forms of MSH(desacetyl-, monoacetyl-, and diacetyl) circulate in the carp and trout blood. Their proportions there are as follows: in the carp, the desacetylated form is 2.5% of total MSH and the monoacetylated form 72% of the acetylated forms of alpha MSH. In the trout, similar proportions were found: desacetyl alpha MSH was 5% of total alpha MSH and monoacetyl-, 70% of the acetylated MSHs.