Mammalian gonadotropins are highly selective. Charge differences between the Cys(10-11) sequence of FSHbeta and LHbeta/CGbeta seat-belt loops determine the ability of these hormones to interact with the LH-R. Selective FSH-R binding is mainly dependent on the presence of an FSHbeta-specific sequence between Cys(11-12) of the seat-belt loop. Intriguingly, African catfish LHbeta (cfLHbeta) lacks a positively charged Cys(10-11) region and stimulates both catfish LH-R and FSH-R with comparable potencies. Our studies on the promiscuous behaviour of cfLH using chimeric gonadotropins revealed that the Cys(10-11) region of cfLHbeta contains cfLH-R-selective determinants, whereas the Cys(11-12) region of cfLHbeta confers FSH-R-stimulating activity to cfLH. Hence, the location of receptor-selective determinants appeared to be fairly well conserved throughout evolution, despite the low sequence identity between mammalian and catfish seat-belt loops. Moreover, various structure-function differences between gonadotropins are discussed in the context of the different (female) reproductive strategies between mammalian and non-mammalian species that required the divergence to a more specific LH-R-stimulating activity of one of the gonadotropins in mammals.