The effects of a low-protein high-carbohydrate (LPHC) diet (8% protein 65% to 72% carbohydrate) were compared to those of regular rabbit chow (14% to 16% protein 57% to 64% carbohydrate) on sex hormone-binding globulin (SHBG) capacity in 12 male rabbits. The six rabbits who were fed the LPHC diet for 8 weeks showed a significant increase in their mean SHBG capacity (mean +/- SE: from 70 +/- 16 nmol/L to 332 +/- 45 nmol/L, P less than .01) whereas the six rabbits fed the standard diet showed a slight decrease (from 106 +/- 22 nmol/L to 76 +/- 20 nmol/L, NS). These changes in SHBG capacity were mirrored by a decrease in percent-free T (from 2.64 +/- 0.26% to 1.64 +/- 0.16%, P less than .01) in the LPHC diet group and no change in percent-free T in the regular diet group (from 2.36 +/- 0.21% to 2.19 +/- 0.10%). The changes in SHBG capacity and percent-free T were not associated with significant changes in testosterone (T), free T, estradiol (E2), thyroxine, triiodothyronine, thyroxine-binding globulin, or cortisol binding globulin levels. In a subsequent experiment, testosterone cyclopentyl propionate (TCP) was administered to six male rabbits while on regular rabbit chow and again after 6 weeks of the LPHC diet. TCP administration did not cause any significant change in the SHBG capacity, but the LPHC resulted again in a significant (P less than .05) increase in SHBG capacity from 80 +/- 18 nmol/L. to 198 +/- 22 nmol/L.(ABSTRACT TRUNCATED AT 250 WORDS)