The bioavailability of 2 slow release formulations (test (P) and reference (R)) of verapamil was investigated in a study with a randomised cross over design, 20 healthy male subjects were included. The mean peak plasma concentration (36.1 +/- 13.1 ng/ml) of the test preparation was reached after 3.75 +/- 0.8 h. The control preparation (R) produced peak values of 34.4 +/- 8.1 ng/ml after 5.6 +/- 3.1 h. MRT (P) was 17.6 +/- 8.2 h and for (R) was 21.1 +/- 7.5 h after (R). The 95-%-confidence interval of the two preparations differed by more than 20%. The release rate of (P) is slower than that of (R).