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976CYANOCOBALAMIN METABOLISM 4ND THYROID FUNCTION (9). Our results with spermatozoa in trial 4 showed a greater T3-I131 uptake by washed spermatozoa and would indicate a possible competition between spermatozoa and seminal plasma constituents or complex diluent constituents for the T3-I131. This points out the need in semen evaluation work for washing the spermatozoa and suspending them in simple salt solutions prior to making determinations of T3-I131 uptake.These preliminary trials indicate that measurement of uptake by washed spermatozoa of T3-Il3I may be useful in semen physiology studies.Summary. The uptake of triiodothyronine-I131 by bull spermatozoa was studied. Trials with frozen semen (-79°C) and stored semen (5°C) indicated that uptake of T3-IlS1 by spermatozoa was related to viability of the spermatozoa. Uptake of T3-I131 by spermatozoa was independent of the amount of T3 added to the diluted semen within the range studied. The composition of the spermatozoa diluent greatly influenced uptake of T3-I131 by spermatozoa. Washed spermatozoa suspended in a 3% sodium citrate dilu-ent had a much greater uptake of T3-I131 than washed spermatzoa suspended in an egg yolksodium citrate diluent, indicating that diluent constituents may compete with the spermatozoa for uptake or binding of T3-I131.