We have previously described the preparation, purification and partial characterization of recombinant (rec) forms of rat luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In the present study, the special functional features of these hormones were studied further, in vitro and in vivo, and compared with human recLH and recFSH, as well as with human urinary choriongonadotropin (hCG) and rat pituitary LH (NIDDK-RP3). In radioreceptor assay, the affinity of hCG binding to rat testis membranes was 5-fold higher than that of human recLH and 100-fold higher than that of rat recLH. In in vitro bioassay, using dispersed adult mouse interstitial cells or a mouse Leydig tumor cell line (BLT-1), hCG and human recLH were 10- to 20-fold more potent than rat recLH. Correspondingly, rat pituitary LH was about 10-fold less potent than rat recLH, and evoked a maximum testosterone response that was about half of that elicited by the other LH/CG preparations. Rat recFSH was about 10-fold less potent than human recFSH in stimulating cAMP production of a mouse Sertoli cell line (MSC-1) expressing the recombinant rat FSH receptor. The circulating half-times (T1/2) of rat and human rec hormones were assessed after i.v. injections into adult male rats rendered gonadotropin-deficient by treatment with a gonadotropin-releasing hormone antagonist. A novel immunometric assay was used for the rat FSH measurements. In the one-component model the T1/2 values of rat and human recLH were 18.2 +/- 1.9 min (n = 7) and 44.6 +/- 3.1 min (n = 7) respectively and those of rat and human recFSH were 88.4 +/- 10.7 min (n = 6) and 55.0 +/- 4.2 min (n = 6) respectively; the two-component models revealed similar differences between the rec hormone preparations. Collectively, rat recLH was eliminated significantly faster from the circulation than human recLH (P < 0.0001). In contrast, the elimination of rat recFSH was significantly slower than that of human recFSH (P = 0.02). In conclusion, rat recFSH and rat recLH display lower biopotencies per unit mass than the respective human hormones in vitro, and also in vivo for LH. This is paralleled by shorter T1/2 of rat recLH than the respective human hormone in the circulation, whereas human recFSH has a shorter T1/2 than human FSH. The special functional features of the rat rec gonadotropins emphasize the use of these preparations on studies of gonadotropin function in the rat, an important animal model for reproductive physiology.