Abstract: In this investigation lifetime performance records for 4633 Holstein cows in northwest of Iran, with opportunity for up to 8 freshening, were used to study lifetime Relative Net Income (RNI) and Relative Net Income Per Day of productive life (RNIPD). A sensitivity analysis was performed in order to determine the relative impact of milk price, feed and non-feed costs on RNI and RNIPD. Total actual milk production had the highest correlation with RNI and RNIPD (0.97 and 0.67, respectively). There were negative correlations between milk fat percentage and RNI and RNIPD which were the consequences of low price for milk fat in the population under study. Negative correlation was found between RNI and RNIPD with age at first freshening. Generally, correlations of studied variables except first lactation milk production with RNI were higher than their correlations with RNIPD. Changes in prices affected the means for RNI and RNIPD dramatically. Increasing the milk price and decreasing the costs resulted in raising means and standard deviations of RNI and RNIPD. The mean of RNI and RNIPD were increased by 2.6 and 2.78% as a result of 1% increase in milk price. Increasing feed and non-feed cost prices caused decreasing RNI and RNIPD. RNIPD was more sensitive to price changes than RNI. Milk price has the most important impact on RNI and RNIPD and then stand feed cost and non-feed, respectively. Correlations between RNI measured in different situations simulated by altering prices were above 0.99. The same trend was observed for RNIPD. Altering the prices had little effect on relationships between various profits functions measured in different situations. Changes of unit prices had little effect on the rank of cows according to their lifetime profit. It was also concluded that RNI and RNIPD were relatively resistant to any effect of possible mistakes due to improper economic assumptions as far as cow ranks are concerned. Correlation between RNI and RNIPD was high (0.79). Examining the result of altering the unit price of different economic items showed that raising the milk price would compensate any unfavorable result of increasing cost prices (feed and non-feed costs) and consequently could stabilize the mean life time profit of a population.