Nicotinamide was shown to inhibit deoxyglucose uptake in three diverse differentiated cell lines. In 3T3-L1 fat cells, nicotinamide equally inhibited basal and insulin stimulated deoxyglucose uptake. Inhibition by nicotinamide was non-competitive. A variety of inhibitors of ADP-ribosylation blocked deoxyglucose uptake while some analogs with no activity against ADP-ribose synthetase also had little effect on deoxyglucose uptake. These findings should be taken into account when inhibitors of ADP-ribosylation are used with intact cells.