Serum concentrations of prednisolone have been measured by radioimmunoassay in 19 healthy volunteers after ingestion of 20 mg prednisolone as four 5 mg tablets. Thirteen subjects were fasting, and 6 had taken a light breakfast. Repeat studies were performed with coadministration of antacid in the 6 non-fasting subjects. The maximum serum concentration of prednisolone was higher, (mean Cmax; fasting: 597 +/- 101 ng/ml, non-fasting: Cmax 489 +/- 56 ng/ml) and the rate of absorption more rapid in the fasting than in the non-fasting subjects (absorption t1/2 fasting: 15.7 +/- 7 minutes, non-fasting: 20 +/- 11.5 minutes). The extent of bioavailability of prednisolone and the rate of elimination were the same in both groups. In the non-fasting groups, the intake of antacid with the tablets had no significant effect on these parameters. This study shows that antacid may safely be coadministered with prednisolone tablets in non-fasting subjects without reducing the rate and extent of bioavailability of prednisolone. It is not known whether the reduction in maximum prednisolone concentration found when the tablets were taken after a meal has any clinical significance.