For studies on the serum lh concentrations of Zebu cattle during the oestrous cycle (Carr, 1971), sequential blood samples were collected from one pregnant cow. A solid phase radioimmunoassay for lh (Goding, Catt, Brown, Kaltenbach, Cumming & Mole, 1969) was carried out on the separated sera and several were found to provide elevated values. As levels of blood lh in pregnant sheep are reported to be consistently low (Geschwind & Dewey, 1968; Niswender, Roche, Foster & Midgley, 1968; Goding et al., 1969), it was decided to collect regular samples for assay until parturition.
The subject was a 5-year-old Angoni (short-horned Zebu) cow in good condition which had calved 9 months previously. Rectal palpation indicated that the animal was 3 to 4 months pregnant at the time that the first blood samples were collected. Blood