African catfish were cannulated in the dorsal aorta to study diurnal changes in blood metabolites. Cannulation of the branchial artery was tested but proved to be less successful. A clear diel fluctuation in the two major blood metabolites, free fatty acids (FFA) and glucose, was observed. Compared to the initial value at 8.30 a.m., the plasma FFA levels dropped by ca. 50% within 2 hours, after which the FFA concentration stayed relatively constant. Minimum values of 0.26±0.04 mM were reached at 12.30. The FFA concentration recovered to the initial value within the following 3 hours. The fluctuation in plasma glucose levels showed a comparable course but there was a phase-shift by 2 hours. The most astonishing finding of our study was the almost complete absence of glucose in the plasma of African catfish (0.05 ± 0.03 mM), a never reported phenomenon for any fish species. This study demonstrates the relatively low level of control of plasma glucose levels as compared to plasma FFA levels in African catfish.