The effects of starvation, refeeding a diet high in carbohydrate, administration of glucagon and cyclic AMP, thyroidectomy, and adrenalectomy on transcription of the gene for liver L-type pyruvate kinase and on the accumulation of cytoplasmic mRNA for L-type pyruvate kinase were investigated in rat. Transcription of the gene was undetectable in either fasted or protein-fed rats. Refeeding fasted rats a carbohydrate-rich diet stimulated an increase in L-type pyruvate kinase mRNA, preceded by an increase in the gene transcription. Transcription was maximal at 12 h of refeeding, decreasing to 10% of maximum at 72 h. The level of L-type pyruvate kinase mRNA remained constant at 50% of maximum for at least 120 h. Neither thyroidectomy nor adrenalectomy affected gene transcription in fasted rats refed the carbohydrate-rich diet, despite a decrease in mRNA abundance to 40 and 20%, respectively, of controls fed a normal diet. Glucagon or cyclic AMP totally blocked the increase in transcription of the L-type pyruvate kinase gene caused by feeding a carbohydrate-rich diet to previously fasted rats. Nevertheless, the level of L-type pyruvate kinase mRNA remained high for 3 h after glucagon administration. After 3 h, the mRNA decreased rapidly with a half-life less than 1 h. Thus, expression of the gene for L-type pyruvate kinase is regulated at both transcriptional and post-transcriptional levels. The transcription is regulated by two major effectors, one positive, namely carbohydrates, and one negative, namely glucagon (via cyclic AMP). Both agents probably act at the level of the mRNA stability as well. Glucocorticoids and thyroid hormones do not regulate transcription of the gene for L-type pyruvate kinase but do appear to be required for a normal accumulation of the transcripts in the cytoplasm.