The aim of parenteral nutrition in tumour patients is to offer an alternative nutritional support to the patient without accelerating the growth of the tumour. For this purpose we fed a total of 100 rats, divided into five groups of 20 animals each (10 with and 10 without tumours), for a total period of 15 days with various nutritional regimes. Group 1 received glucose, group 2 long-chain triglycerides, group 3 medium-chain triglycerides (MCT), group 4 omega-3 fatty acids, and group 5 an oral diet. On the 10th day the Yoshida sarcoma in its ascites form was implanted into the tumour-bearing rats. In animals receiving MCT or omega-3 fatty acids tumour growth was considerably smaller than in the other groups (group 1 vs. groups 3 and 4; p < 0.05). Unfavourable effects of the administration of these fatty acids on the general condition of the animals were not observed [muscle nitrogen content (mg/kg body weight): MCT = 82.3, omega-3 fatty acids = 65.25]. The impulse cytophotometric measurements did not demonstrate any influence on the pattern of cell division (p > 0.05). We think that modulation of the immune system by feeding with MCT or omega-3 fatty acids was responsible for the reduced tumour growth in relation to the other groups. The extrapolation of these results to the clinical situation, however, may not be possible.