In order that the luteotrophic activity of luteinizing hormone (LH) in the dog might be more precisely elucidated, LH and progesterone were measured in blood samples collected at intervals of 10-15 min over 360 min from three beagle and two foxhound-boxer bitches at a defined day during pro-oestrus, the phase immediately after ovulation, early and late dioestrus and early anoestrus. In addition, the three beagle bitches were treated with the gonadotrophin-releasing hormone (GnRH) analogue buserelin at a given day during each quarter of dioestrus and blood was sampled over 480 min at intervals of 15 min (for 2 h), 30 min (for 4 h) and 60 min (for 2 h). Cyclic activity was monitored by determining progesterone at intervals of 1 or 2 days up to 140 days. In spite of some apparently breed-specific differences, the results indicated that the availability of LH depended on the stage of the cycle. The area under the curve (AUC) was higher during pro-oestrus, late dioestrus and early anoestrus (P < 0.01-0.05), largely as a result of an increased pulse amplitude and not of changes in basal concentrations or pulse frequency. After GnRH the AUC for the provoked LH release increased from 1206 ng (150 min) ml-1 (first quarter) to 4045 ng (150 min) ml-1 (fourth quarter) (P < 0.01); maximum LH values were reached between 25 and 40 min. In neither experiment did progesterone increase during the observation periods of 360 and 480 min.(ABSTRACT TRUNCATED AT 250 WORDS)