A commonly used and specific plasma LH radioimmunoassay was modified to provide reliable results for cow LH concentrations in blood within 4-5 h. Blood samples were collected at 2- or 4-h intervals from at least 24 h before the expected oestrus until after its onset. Thereafter, samples were collected at intervals of 2 or 4 days until the next oestrus. For the assay, the antigen-antibody reaction took place during a 2-h incubation at 37 degrees C. The immune complex was precipitated by addition of rabbit anti-gamma-globulin and polyethylene glycol followed by an incubation at 22 degrees C for 30 min. The coefficient of variation between this assay and the standard assay was less than 15%. From a study of 25 cycles from 12 animals, we obtained intervals of congruent to 27.3 h between the beginning of the LH peak and ovulation and congruent to 17.5 h between the end of the peak and ovulation. This modified technique can be used to predict rapidly and precisely the time of ovulation in cattle.