Biotin bioavailability from poultry and swine feed ingredients was determined in an experiment involving growing rats (55-60 g body weight, initially), housed individually in stainless steel cages with raised metal floors. The rats were fed a biotin-free diet fortified with egg-white powder for 7 d prior to being put on test. Thereafter, 5 rats were randomly assigned to each of the experimental diets, as follows: a basal egg white-free diet (A) without added biotin, or supplemented with graded levels of d-biotin, i.e., 0.05, 0.10, 0.25, 0.50 and 1.00 microgram/g, and also 12 test diets prepared by incorporating various cereal grains and/or two protein supplements into diet A by partial replacement of casein and carbohydrates. The experimental diets were fed ad libitum for 21 d, and met or exceeded recommended levels for all nutrients except biotin. Results showed significant correlations between pairs of parameters, including plasma biotin vs biotin intake (P less than 0.01), liver biotin vs biotin intake (P less than 0.05) and plasma biotin vs liver biotin (P less than 0.01). The bioavailable biotin from test ingredients was estimated using the derived regression equation, Y = 0.54X + 1.05, (r = 0.85), where X = biotin intake (microgram/d) and Y = plasma biotin (ng/ml). In most cases, these values were greater than the corresponding biotin intakes, indicating that intestinal biotin synthesis and coprophagy might be increasing the supply of bioavailable biotin to the rats. Therefore, the rat might not be a good model animal for routine evaluation of biotin bioavailability from feed ingredients.(ABSTRACT TRUNCATED AT 250 WORDS)