Pituitary somatotropin (growth hormone) and prolactin are two related protein hormones produced by the pituitary gland. Both contain single polypeptide chains of 190-200 amino acid residues (Wallis, 1978). In mammals, prolactin acts primarily on the mammary glands, promoting mammary growth and lactation; in many species it also has direct effects on the gonads and possibly other organs (Horrobi...

Mammalian prolactin stimulates growth and inhibits metamorphosis of larvae of several species of amphibians. Mammalian somatotropin has little or no effect upon larval growth or metamorphosis unless used in high concentrations. The evidence behind these statements and some of the implications are critically evaluated. Growth of postmetamorphic amphibians is stimulated by somatotropin but not by...