Changes in prolactin (PRL) levels and PRL-producing activities in the pituitary gland of tadpoles during metamorphosis were studied. PRL concentration in the pituitary gland as measured by a homologous radioimmunoassay was relatively low during pre- and prometamorphosis and early climax (2.5-3.5 μg/100 μg protein). PRL levels increased thereafter and reached maximum (7.3 μg/100 μg protein) at late climax (stage 24). It was also demon-strated that the pituitary cells which immunologically reacted with the antiserum to bullfrog PRL increased in number as metamorphosis proceeded. Measurement of incorporation of [3H]leucine into pituitary PRL in vitro revealed that elevation of PRL synthesis took place in the tadpoles which had reached climax. Radioimmunoassay and bioassay of PRL in the pituitary glands of adult frogs and tadpoles indicated that there was no difference in immunological and biological properties between adults and larvae.
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