The objective of this study was to estimate lactation curves for cows of different age at calving in winter and summer seasons. Data consisted of 1 289 522 test-day records of cows calved in Wielkopolska region in 1992-1996. Milk, fat and protein yields in first three parities were studied. Test-day model with 50 classes of days-in-milk by parity was used. Lactation curves were derived from solutions for this effect. They were flatter for first versus later lactations. Peak yield for milk production occurred in the second part of the first month of production. Shape of lactation curves for different age and season classes did not vary substantially. Cows calving in winter season had higher production. Similarly, older cows had higher yields for all traits and lactations, however the evidence of age at calving by season of calving interaction was found.