Acute and chronic studies of male and female meiosis in an Imperial-Chemical-Industries-derived strain of mice were carried out with LSD-25. Over a wide range of doses claimed to be effective in teratogenesis and male meiosis in the mouse and in the production of chromosome damage in leucocyte cultures of human blood, no convincing effect was detected on the chromosomes of first or second meiotic divisions, nor in those of the bone-marrow or of the few spermatogonial metaphases that were studied.