Sonographic analysis of isolation-induced calls of 10- to 17-day-old Sprague-Dawley rat pups showed that average acoustic parameters of calls changed with pups' age. Average call duration increased with age from 80 ms to over 140 ms in 15-day-old pups. The peak frequency increased from approximately 50 kHz to an average of 64 kHz in 17-day-old pups, and the average bandwidth increased from 34 to 45 kHz in 17-day-old pups. Analysis of the sonographic structure of pup calls additionally revealed a tendency to produce two or more alternating sweeps of sound frequency in each call. Development of sweeps is the most typical feature of pup calls with a dominant call type resembling "U" or inverted "U" shape in the sonogram. Number of "U" or inverted "U" call types significantly increased with pups' age. It is concluded that pups developed and strengthened those acoustic features of distress calls which play a role in intraspecific communication and maximize pup survival.