The consumption of algae has increased considerably in recent years. However, the nutritional consequences of the algae consumption have scarcely been studied and there are some indications of serious adverse effects of algae consumption in children and adolescents. This study aims to assess the effects of balanced diets containing 7% algae (Nori or Konbu) for 3 weeks on (i) dietary intake, growth, and dietary efficiency ratio (DER), (ii) absolute absorption of several minerals, and (iii) size and histology of the liver, spleen, kidney, heart and bowel of growing rats. All rats were fed a diet containing 93% casein-soybean oil-base. The remaining 7% of the diet consisted of a cellulose/wheat starch mix (35/65) in control rats, freeze-dried Nori (33.8% fibre) in that of the Nori group and freeze-dried Konbu (36.1% fibre) in rats fed Konbu. Food intake was similar in all groups. Body weight gain and DER, however, were somewhat lower in the Konbu group than the other groups but this effect was not statistically significant. The algae diets had a higher content of most of the minerals studied and with some exceptions alga fed rats also presented a higher absolute absorption of these minerals than control. There was no effect of algae consumption on organ weight except that the spleen weight, whose size appeared to be related to the dietary Na/K ratio, was significantly lower in the Konbu fed rats than in the other groups. Histological analyses did not demonstrate any remarkable changes in the kidneys, spleen or heart. However, the incidence of non-dense glycogen-like vacuole presence observed in livers of the Nori rats was significantly higher (P<0.05) than in Konbu and control rat livers. The Nori fed rats had a higher incidence of submucosal oedema than the Konbu fed rats, which may be related to the higher Na/K ratio in the Nori diet. Konbu rats had a lower score for most of the various histological parameters than Nori and control animals suggesting that Konbu is preferable to that of Nori. Further studies are needed to investigate the positive and negative effects of large-scale consumption of Nori and Konbu algae.