The effect of the hydrocarbon chain of the model 4-substituted 1-(3-chlorophenyl)piperazines 12-31 on their affinity for 5-HT1A receptor sites was investigated. It was found that elongation of the 4-n-alkyl chain strongly increases the 5-HT1A affinity of the investigated compounds. The affinity reaches the maximum (Ki = 2.67 nM) for the n-hexyl derivative 20. It was shown that hydrophobic interactions of N-4 substituents of 1-arylpiperazines significantly contribute to their 5-HT1A affinity. The specific binding constant was defined as the receptor affinity of the protonated species of compounds under physiological conditions. The range of Ki(AH+) = 1 - 3 x 10(-11) M is a specific affinity limit of the investigated class of compounds at the 5-HT1A receptor sites.