To investigate the effect of simultaneous glucose and galactose administration on serum galactose levels in man, volunteers were given a standard galactose meal of 0.5 g galactose/kg BW alone and with various body weight related glucose loads and with fructose; lactose was also given to a group of volunteers. Two groups of subjects received the standard galactose meal alone and with a simultaneous intravenous infusion of glucose or insulin. There was a marked individual variation in the serum galactose response to the standard galactose meal, the maximum galactosemia ranged from 0.23 to 4.56 mmole/L. Peroral glucose suppressed the serum galactose response to galactose producing significant reductions in the mean area under the serum galactose response curves. At a glucose intake of 0.15 g/kg by-32 +/- 14.3%, 0.50 g/kg BW -69 +/- 5.93% and at 0.75 g/kg BW -75 +/- 4.93%. Ingestion of glucose with galactose did not increase galactose loss in the urine. Lactose produced similar serum galactose, glucose and insulin responses to those seen after administration of equal quantities of galactose and glucose as monosaccharides. Fructose did not affect serum galactose levels when given with the standard galactose meal. Intravenous glucose produced a significant reduction of 56 +/- 14.1% in the mean area under the galactose response curve [p less than 0.01], whereas intravenous insulin did not affect the serum galactose response to peroral galactose.