We have examined the mechanism of the carbohydrate induction of rat hepatic mRNA-S14 to the intragastric administration of sucrose and its constituent hexoses, fructose and glucose. A maximal sucrose response (26-fold) was obtained with 2 ml of 60% sucrose/100 g of body weight, yet equimolar concentrations of fructose and glucose were not able to reproduce the response. Fructose yielded less than one-third the response observed with sucrose, whereas glucose administration was ineffective. On the other hand, the simultaneous administration of both hexoses restored the full response observed with sucrose. The synergism between fructose and glucose was not due to the administration of extra calories or enhanced insulin release but was correlated with altered carbohydrate metabolism as reflected by the hepatic pyruvate content (0.058 +/- 0.003 mumol/g fasting, 0.067 +/- 0.009 glucose treated, 0.218 +/- 0.030 fructose treated, 0.292 +/- 0.015 sucrose treated). The time course of induction of mRNA-S14 nuclear precursor in response to fructose, glucose, or sucrose correlated with the mature mRNA-S14 values. In addition, enhanced transcriptional activity of the S14 gene was observed after sucrose feeding (from 25 +/- 8 ppm baseline to 108 +/- 40 ppm at 4 h) and fructose feeding (40 +/- 21 ppm at 4 h). These studies demonstrate that the carbohydrate regulation of mRNA-S14 is due to a synergistic interaction between fructose and glucose reflected in the hepatic pyruvate content and leading to the induction of the nuclear precursor with an associated increase in transcriptional activity of the S14 gene.