The effects of several alkaloids on the invertase activities of Ricinus communis, Solanum tuberosum, Oryza sativa and Carica papaya were studied. The following alkaloids: caffeine, berberine, strychnine, morphine, ethyl-narceine and nornicotine, have different inhibitory effects on invertase activity. Kinetic studies of the alkaloid inhibition revealed that different alkaloids exhibited diverse types of inhibition of invertase. It was found that alkaloids form reversible complexes with invertases. However cocaine and potato glycoalkaloids were activators of some invertase. The sugar moiety of potato glycoalkaloids were also activators. It is concluded that alkaloids may be effectors of higher plant invertase activity.