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Chamilová M., B. Bojková, K. Kalická, P. Kubatka, E. Adámeková, E. Ahlers- ová, I. A hlers, B. P ástorová: Retinyl Acetate and Melatonin in Chemoprevention of Mammary Carcinogenesis in Female Rats: Metabolic Changes. Acta Vet. Brno, 2001, 70: 57-63. Combination of chemopreventive substances is expected to be an effective way to combat cancer in the near future. We therefore tested the effect of retinyl acetate (RA) and melatonin (MEL). N- methyl-N-nitrosourea (MNU) was used to induce mammary carcinogenesis in female Sprague- Dawley rats (n = 71). NMU was administered intraperitoneally in two doses, each of 50 mg/kg b.w. between postnatal days 43 and 57. MEL was given in drinking water (20 µg/ml) to group 1 (n = 18) for 22 weeks, beginning 12 d before NMU administration. RA administration (8.2 mg per animal and day per os) in group 2 (n = 18) started 11 days before NMU administration and lasted for 22 weeks. Group 3 (n = 17) received both chemopreventive substances and group 4 (n = 10) was left intact. The incidence, latency period, frequency and tumor volume were evaluated in all animals. Selected indices of carbohydrate and lipid metabolism were evaluated in tissue samples and blood serum. RA+MEL combination decreased the incidence and frequency per group and lengthened the latency period for tumor occurrence. Both MEL and RA+MEL decreased the serum glucose concentration in tumor-bearing (TB) as well as in non-TB (NTB) animals. In TB animals a decrease of serum cholesterol (CH) in comparison with NTB and intact animals was observed. In the liver of both TB and NTB animals, RA administration led to CH accumulation, RA+MEL combination did not influence this situation. MEL alone and in combination with RA markedly decreased the malondialdehyde concentration in bone marrow of both TB and NTB animals. The combination of RA + MEL extended the possibilities of a succesfull prevention of experimental mammary carcinogenesis. MEL did not cause substantial side effects in the chronic administration protocol and the decrease in tissue lipid peroxidation predominated among its effects.