Concurrent falls in milk production and electrical resistance of composite milk were examined in Israeli Holstein cows. The cows were milked three times a day by a system that recorded yield and the lowest electrical resistance in the composite milk from the four glands. The study included two groups: cows that experienced on day 0 a decline in resistance and milk production [ges ]20% from the mean of the previous 9 d (62 cows, case group) and cows that experienced no such episodes over 9 d before and after a fixed day (118 cows, control group). Bacteriological status and somatic cell count (SCC) or California mastitis test scores were assessed on the fixed day in the control group, and on days 0, 1 and 2 in the case group. California mastitis test scores greater than 2 and SCC thresholds of 5×105 cells/ml were used to create two classes of leucocytosis. There was no statistically significant difference between the two groups in frequency distributions of pathogens and their types: in 30% of cows infection was not detected, 33% were infected by major pathogens (95% of which were Staphylococcus aureus), and 53·5% by minor pathogens (80% Micrococcus spp.). Cows in the case group had lower milk production during the 8 d following day 0. Mean electrical resistance was lower in infected cows and particularly in cows infected by Staph. aureus. High leucocytosis was associated with reduced electrical resistance in both groups, and was found in 93% of cows in the case group v. 25% in the control group. The results suggest that falls in electrical resistance of milk and in milk production were not linked to a specific pathogen, and were followed by 3–8 d of reduced milk production and electrical resistance. The study suggests that there are episodic aggravations in mammary health that do not evolve into clinical mastitis but may induce significant losses in milk yield and quality.