A study was conducted to investigate the effect of feeding diets containing alfalfa, nori or pine-needle meal on cecal size and cecal unease activity in male and female rats. Six male and six female Sprague Dawley rats (initial mean weight, 92 g) were assigned to each of the following diets : a control diet or diets containing 10% alfalfa, nori or pine-needle meal. All the diets contained 5% corn oil and 5% lard. After a 4-week feeding period, rats were killed and cecal contents and blood samples were collected. Rats fed the control or nori meal diet had higher (P$lt;.01) average daily feed intake(ADFI) and average daily gain (ADG) and lower feed/gain ratio (F/G) than rats fed diets containing alfalfa and pineneedle meals. Male rats showed much higher (P$lt;.01) average daily gain, and lower (P$lt;.01) F/G than females. Ammonia concentration (μ㏖ ammonia per g collected contents) in the cecum (blank or before incubation) was higher (P$lt;.05) in the control than in the other groups. Urease activity in a g of cecal contents of rats fed diets containing nori or pine-needle meal was much lower (P$lt;.01) than that of rats fed the control or alfalfa meal diet, but was not different between sexes. However, total urease activity in the contents was not significantly different between the dietary treatments. Interestingly, male rats showed higher (P$lt;.01) total urease activity in the contents but lower (P$lt;.01) plasma urea concentration than did female rats. Results of our study indicated that dietary nori or pine-needle meal increases cecal size and decreases cecal urease activity perhaps due to soluble fiber present in the ingredients. The higher total urease activity in the cecal contents of male than that of female rats is interesting, but its mechanism is yet to be studied. The effect of sex on blood urea concentration is considered due to different amounts of protein intake per unit body weight.